Wikis are great for reference. But to rely on, you gotta play the game. One GTA wiki site states that 72's lead character is named something, but in game (and another site) states him as Judd Parker. I've had good experience with Fallout Wikia, but your firsthand knowledge is invaluable. No wikia can substitute that.
Take COL or JIF - much of that success comes from tying it with IV - and that's because i knew the game so well. I'm working on a GTA V one, and that's a little harder because i feel less attached to GTA V than IV. Fallout is a good example - there's a lot of lore there, and it's difficult to tie it in with the story you're writing.
Reaching Fanfics is easy - your own gameplay, youtube vids (cutscene and gameplay for quick reference), online game-scripts, wikias - you have all these tools at your disposal, and an already-established world and characters.
Original works are much harder. There is no ready-made world, no pre-developed characters. You gotta do it all. Sure you could base it in NYC, where you might already have knowledge of but you've got to do all the work.
The upside is that FF much of the work's done for you, and with originals you can create something tailor made for your story and something epic.
The downside is that with FF you got to get it spot on perfect - something i failed with in Fallout (which is why i think I'm receiving little interest in it atm). You've got to nail that feeling from the games (or film etc), nail the characters and make the reader feel they're experiencing something "official". With originals you've got to do all the legwork, and missing out even a tiny thing can f*ck everything up.
Certain FFs rely on atmosphere - and Fallout can be considered in that catagory. Others are built of small details, so it depends what you're researching.
what are you researching (if you can say) or are you just asking generally? If it's a in depth, gritty noir about a chicago cop in the prohibition era, you'd have to delve into the history books in your library, watch old Al Capone movies, and lots of noirs. If it's a technical war story, you'd have to research war, tactics and politics. Tom Clancy, perhaps? A story about heists, you'd have to research them too - read news reports about famous heists, look at those in say GTA V and how they wouldn't work etc.
Researching the world is a key point - I once wrote one set in NYC and LA, and spent a lot of time on google maps. Street view. You need to know the physical setting, but the era too. Then theres characters - you're own creations, often, you've got to develop them (and i find that difficult. I'd love some tips on character developments). Then there's technical details like guns, calibers and ballistics; cars and mechanics etc.
It can be daunting and often people overlook it, but that often shows. I haven't written originals for a long time. Nikolay's Nightmare was the last one i tried and no one took any interest in it, so it died with a whimper. I must confess that knowing FFs will get more interest has pulled me in to them more, so i write them. After Fallout I might pick up my AC one, and there's GTA V one too. But even though people will see "GTA V Fanfic" and be interested, it doesn't mean you can just sit back. You put as much effort in FFs as you do in originals, and it all starts with research and prep.
Don't rely on anything other than the game, or film - the source media. Quick fact-checking, use youtube or wikias, but only to remind you of what you already know. Playing through GTA IV, absorbing every little detail is what made COL good. I'm proud of it, i worked hard on it, and i still had to put an effort it, but it only worked so well because of the groundwork the source game had already laid. Your own knowledge of the game will be more useful than any number of wikias.