That's really hard. I'm going to recommend some of my adventure favourites, but none of them really have the same feel as GF.
There are three games in the series. The best way to describe them is to say that they're interactive supernatural murder mysteries. They have interesting back drops (Voodoo in the first game, Bavarian history in the second and The Templars in the third), so that might interest you a little. The music is excellent, but not similar to that in GF. GK has a more gothic soundtrack with additional pieces that suit the current location, like African drums or blues in some places of the first game. You could say that Gabriel is a little like Manny in the sense that in the beginning they're both failures and they eventually rise from there, but don't expect to see Manny the II
. GK games also have their dark moments with just enough humour somewhere in there.
Beneath a Steel Sky:
At first you could say that this game is nothing like GF. It's a scifi adventure about a futuristic world where the society is controlled by a computer called LINC. However, the basic story is the same as in GF; a random guy is caught in the middle of it and must reveal the corruption in the society. It's also freeware, so you won't lose anything if you try it. It's rather old, so prepare yourself for old graphics and midi soundtrack. Has full voice work, though. You can download it here.
Unfortunately, both are point and click, so you may not like them.
By the way, I'm moving this to Calavera Cafe since we're mostly going to be talking about non-GF games.