Well, there's no C coding in either type of projects. Both Fallout 2's engine and Gamebryo use their own scripting languages. The only times C or any other common programming language is used for projects like this is when programing new modding tools to be used (Script Extenders or software to extract and pack specific file types for example).
Also you need a huge amount of different skill sets in both of these projects. I mentioned before some of them, but here's a larger list:
It would be needed scripters, 3D graphic artists, 2D graphic artists, animators (making new animations for NPCs, characters, enemies, environment, etc) quest designers, dialogue makers, voice actors (if needed), character/npc makers, sound creators and editors (music, sound effects, voice files, etc), mappers and world builders (to make the maps, like towns, dungeons, the entire world, etc.), script extender coders (they make new functions to be used by script extenders for the games), proper testers (and not just people who want to play the game before it's publicly available), and others that I can't even think about at the moment.
Then, if you manage to get people with all the skill sets you need for the project, you will need a proper team structure:
You need a proper project manager, you need good and reliable communication between all the team members, you need a proper plan on how the project should advance, you need a good way of members to submit their updates without breaking or conflicting with updates submitted by other members, you need the team to be motivated and willing to work on the project for years (if needed), you need to keep the team together through the bad times (and the bigger the project, the more problems will happen). You need to be able to find good replacement members, because it's likely members will have to leave the project (the longer the project goes, the higher chance of people starting to leave).
You need time, a lot of time. You can't think that it will just take an year or two. Because these projects will most likely take one or several decades.