That falls into a very grey area of Copyright and Intellectual Property law. Generations Arena is legal to distribute for the following reasons:
1) All the characters we're using already exist within Quake 3 Arena. Character trademark can be very touchy, and it depends on the disposition of the holder of the intellectual property, in this case, Id Software.... however, things have changed a bit since we got started. Id Software is now owned by Bethesda, so while Id greenlighted our project, Bethesda may or may not wish greenlight a similar project even if Id is agreeable to it.
2) Our models, textures, maps, etc, are all original content, or in the case of content such as maps or sounds that are not our original content they are used with permission. The weapons resemble and function like weapons from other games, but the code that drives them is our own. In the non-public beta there are particle systems heavily derived from Quake 1 and Quake 2. This code is governed under the GPL license, along with the engine source to Quake 3 Arena. That means we can do this so long as we make the source code to the engine available. Notice I say engine, NOT the game logic. We never moved the game logic over into the GPL'd game source logic that accompanied the engine. The game logic is governed under the original Q3A 1.32 Point Source license, which means while the engine will be open source the game logic can remain closed source. That helps to make it more difficult for players to cheat.
3) All of this was discussed with Id Software PRIOR to our start on Generations Arena, and Id Software gave us explicit permission to put this project together.
That's why Generations Arena is a legal project, whereas other projects with a similar vein have not been. All of the intellectual property for the characters is Id Software's. Our content falls under our own copyright, since we've made the models, textures, etc.
Now, why the long dissertation? Releasing a Generations-ish mod without Id's blessing might run afoul of their intellectual property rights as you'd be taking characters from other games and placing them inside GZDoom. This runs into the history of the old Generations mod for Quake 2, which unfortunately, requires another long dissertation.
Generations for Quake 2 was designed to put the shareware episodes from Doom, Quake 1, and Wolfenstein 3D into Quake 2. You could play as any of the four characters - BJ from Wolfenstein, Doomguy, Quakeguy, or the Quake 2 guy. You could feasibly play through the entire shareware episode of Quake 1 using the Wolfguy, Doomguy, or Quake 2 guy. Everything looked exactly like Quake 1 since the assets from Quake 1 were used, and therein was the problem. Skid, the original programmer for Generations Q2 thought he had permission to do all this. Well, Skid eventually left the project, and handed it over to Lee'Mon. Lee'Mon liked the concept, but had some misgivings about its legality shortly after taking over. After some emails exchanged with Id to clarify things, Lee'Mon was left with three options: Redo Generations Q2 from the ground up with all original content, take the project underground and continue it without Id's permission, or cancel the project. Reluctantly, Lee knew that option 3 was the only real choice, and that's what happened. Generations for Quake 2 was scrapped, and the source code was made public.
Not everyone was happy about this though, and some fans of Generations Q2 decided to try to continue it on their own. This resulted in two projects - one that took the project underground and continued it illegally, and the other that tried the legal route. The first project was originally called iGen, later renamed to RSRC, or Resurrection. This project eventually fell off the map and was never completed or released in any kind of stable form. The second project was SOG, which stood originally for "Save Our Generations", and later, "Some Old Games". This was the "legalized" release of Generations for Quake 2. I'll reserve personal opinion of either project to focus on what was required of the latter. SOG had to rename all the characters, create all new maps and environments, and all new textures. They did the project the "right" way legally in that things had a look and feel of the old games, and no legal boundaries of copyright or intellectual property were crossed.
While this was going on, Lee'Mon was not finished with the Generations concept. With the release of Quake 3 Arena, which contained all the original characters (Sans BJ, so we had to kind of invent Sgt. Blaze to be a "stand-in" for BJ, using the Sarge model) within a multiplayer deathmatch setting, Lee saw a golden light of opportunity. He sent inquiries in to Todd Hollenshead, then CEO of Id Software, explaining what he had in mind, inquiring about the legality of it, and finally receiving Id's blessing. After a rocky start, I joined the Generations Arena team after working on a side project that caught Lee's attention. Shortly after, the project's coder went AWOL, leaving a void. I knew a fellow named Warden from the old SIN clan from when I was playing Quake 2, and some other individuals interested in Generations, and we formed a new team to work on Generations Arena. In the process I began to learn how to program in C, and after a few months when Warden was unable to continue helping as the primary coder due to real life concerns, I took over as the programmer while he still provided us with webhosting services, a game server, and an IRC server. We recruited one hell of a talented texture artist in the form of Tabun, Renalicious and HedHunta helped to throw some gun models together in addition to my own, ReBoOt, Mykul, SpAwN (Now known as Thomas Mink) put together some kick-ass maps, and we found a bunch of other good maps on LvL world, ConfusedUs, Kenny.exe, and Doktor Jones helped with our site design, and Lee'Mon guided us and kept us legal and provided the glue that helped keep the team together, and several beta testers, including and not limited to Lilazzkicker, Angs7, Makou, and Vampira, have helped test and squash some rather nasty bugs. I'm sure I left a lot of people out - they're in the credits - but that's how Generations Arena became a reality... if still a work in progress. Most of the team has had to move on to other things, but I am blessed with being able to continue work on the project, so as long as I have internet and the data isn't lost things will continue.
To release a Generations-ish project for Zdoom, that's the path that would have to be followed. Weapon models and textures, sounds, maps, etc would have to be original content or used with permission. The characters would need to be renamed and original character models (or sprites) created, and if any code was duplicated it would have to be GPL. If the code differed enough to be considered original, that doesn't have to be considered GPL, but this is kind of fuzzy area. In our case, Q3's split engine, server, and client logic is so radically different from that of Quake, Quake 2, and Doom that we had to figure out how to achieve the similar results using very different logic. For example, you can't copyright 1 + 2 = 3, since that's a basic mathematical concept. How you GET 1 + 2 to equal 3 in your code can be another matter. Wolfenstein we didn't have to worry about since Q3 and Wolf are so drastically different that nothing could even be considered close code-wise. That complexity is also a huge reason behind why Generations has taken so long to program. It's ridiculously complex. It's not just a matter of having 46+ guns and 5 sets of movement physics. It's having at least 36+ of those guns that look, feel, and act like they did in other game, plus 4 sets of movement physics where at least 3 of them have to look, feel, and act like they're from other games - and all of this has to work together and be balanced, and not have serious bugs. I think I devote about 5% of my programming time to actual programming, and the other 95% to testing and debugging. That's why Gen is not severely buggy compared to alot of other projets.
There's one other minor problem with releasing a Generations-ish mod as well, and that's the second part of intellectual property concerns - in this case ours. While the characters are Id's intellectual property, Generations as a concept is technically Wirehead's intellectual property, but not in its entirety. Quake 3 Arena put all the characters into one universe and game, but it's Wirehead that's taken the guns and physics from those games and put them together. That complicates the matter, as while we're primarily concerned with not violating Id's intellectual property and copyright for our own project, we're the only team that's been given direct permission to use their intellectual property in this manner. That means another project using the same idea with the same characters within the same serious of games could technically be violating Id's intellectual property rights by proxy while at the same time violating ours. What this means is, while we might be able to tell someone else it's OK with us to use the Generations concept or even call it Generations, it might NOT be OK with Id Software. That puts the prospective modder - in this case MadTux - at the same juncture we were at before Generations Arena was conceived as a project: Needing Id's permission and clarification to proceed.
I know that's a pretty large wall of text, but Lee'Mon researched the complexities of all this long before Generations Arena was green-lighted. If not, Id could have sent a cease and desist letter before anything got off the ground. That's what happened with the Reborn mod for Q3. The goal of that team was to use some high-quality 3D models of the Doom monsters and re-create the game play of the original Doom game from within Quake 3 Arena. Even though the content was going to be all original, the intellectual property still belonged to Id, and at the time, Doom 3 was on the horizon. Id sent their team a cease and desist letter, which they did comply with. That's the kind of mistake we don't want to see someone else make - to put something really awesome-looking together, spend a lot of time and effort making it fantastic, and then find out that it's all for nothing.
So to do this there's a lot to think about, and there's also a hell of a lot of work involved. Keep in mind Quake 3 Arena was released in 1999, Generations was announced shortly after, and we're still working on it 15 years later.