This is something I was wondering about today when I realized that we still don't have Vampire: The Masquerade: Bloodlines. What, exactly, are the requirements for a game to be released on GOG?
I know you need a publisher's, and ideally also the developer's, consent. Any given game also needs to be reworked up to the point where it can reliably function on modern computers. But is that all there is to it?

Price. A game has to already be within GOG's price range (or ready to be marked down to it) before it can be released here. Bloodlines still retails for $20.
Also, its a bit more than just getting a publisher's "consent". After GOG has gone through the potentially grueling process of finding the correct rights holders and negotiating with them, GOG enters into a business contract with the respective companies, agreeing to pay them a fair percentage of the sales while also keeping a fair percentage for themselves. That can take months to accomplish before they even get to the point where they can start "reworking" a game to run on moderns systems.

oh, and don't forget the ritual sacrifice to the god of GOG.