[RPG] reasonably complete database of RPGs

In terms of the GURPS part, assuming this is not for sale and just posted publicly on the Web, your use would be dictated by the Steve Jackson Games Online Policy. This allows you to make adventures and stuff but not things that require a restatement of the GURPS rules - so you'd want to be careful that your "fan book" doesn't do that.

In addition, the online policy warns you about the second problem -

Post GURPS rules for a book, movie or TV show?

Be careful here. Even if what you do is completely within these guidelines and does not infringe Steve Jackson Games, you're probably infringing the copyright of the creator of the book/movie/TV show. They have the right to decide what use gets made of their intellectual property, too. The better known the property, the more likely you are to get a letter from a lawyer. But even with something obscure, courtesy dictates that you get the permission of the creator first. Then, if you have any doubts about your use of OUR material, ask us.

Making a fan book for those video game properties is technically not legal (though they may have a fan use statement of their own that lets you). However, many netbooks exist out there in the wide world because of a mix of flying under the radar/no one caring.

Super Legal Answer:

I am not a lawyer, hire one to advise you! The most "correct" answer from the point of view of a cover-your-ass legalistic society. They'll just tell you not to do it, though. "Fun versus a minute amount of legal risk? What is this 'fun'? Don't do it or you may end up in PMITA federal prison. That'll be $500." Save the money and just don't do it if you're risk averse.

Semi Legal Answer:

Contact both SJG and the game IP owners asking for permission to do this. Of course, it'll be hard to actually contact anyone about those old games, and the routine answer is "no" for more of those CYA legal reasons above. Sadly most media companies haven't figured out yet that stuff like this just helps to increase the buzz around and value of their brands.

The Most Common Answer:

Do it, but be prepared to take it down if any of the parties involved demand you do. Some may. Carries marginal risk of a vengeful company trying to ruin your life even if you take it down, which best as I know has never happened over a free fan book.

While I'm not a lawyer, I'd suspect that part of your liability here is determined by the method you do the 5e updates. Are you talking about republishing them, including maps and flavor text? If so, I'd be very cautious, especially given WotC's proclivity to shut down 3rd party resources for older versions of the game.

However, a simple listing of changes gives you a much larger footing here. Many of these changes should revolve around monster and trap changes, so I'm thinking something like

p. 2, Kobolds become 5e Kobolds (MM p.whatever)
p. 3, Greater Golem becomes Iron Golem (MM p.whatever)

and etc.

While Wizards might still send you a cease and desist, it seems an awful lot less likely. The maps and original text are almost certainly under copyright, so I'd be extremely cautious about republishing them.