Please add *insert game/publisher here* to the GOG library!
GOG has a handy dandy wishlist that you can use to post your requests for games on GOG. Just make sure you search the wishlist to see if anyone else has already added the game you want and if they have, vote for it instead of creating a new entry. Additionally, if you wish to simply discuss the games you want to see on GOG, you can post in the Official Beg Thread: "This Game Belongs on GOG!".

Will you be adding *insert game/publisher here* to the GOG library?
GOG does not release specific information on prospective games or publishers until they already have agreements in place. They will not comment on who or what they are trying to get, only what they do have. Check the site's main page for news on what is coming soon, it gets updated every Thursday.
Why are some highly requested games, such as System Shock 2, not being added to the catalogue?
GOG tries to get every game they can, but sometimes it is a difficult process to find the correct rights holder, secure permissions to sell the game, get gold masters of the games, modify as necessary to make the game work on modern OSes, etc. It's not that GOG is intentionally not adding a highly requested game, its that they can't unless or until they can meet all those requirements. That certainly doesn't mean you shouldn't keep requesting and voting for games, though. The more interest we can show, the more "ammunition" GOG has to bring to rights holders to convince them to allow their games to become part of the GOG library.