Games i cant gift here at steamgift

Thats because some of that games have been given away for free in the past (like Pixel Puzzle Japan in IndieGala for eg). Don't know if there is a list around somewhere, guess you just have to try creating the GA to find out.

Well, for one, you can't giveaway DOTA2.

But seriously, I don't think currently there is an official list, so the unofficial one will have to do.

Also, for your purpose, you can just click "Create Giveaway", start typing a game name, and if the game name doesn't pop up you'll know it's been disabled.

It's unofficial but here you go: http://www.steamgifts.com/discussion/pJRbR/can-we-have-a-list-of-games-that-cannot-be-created-as-giveaway-unofficial-list-inside

The list of games you can't giveaway is basically the list of games you can giveaway on the giveaway creation page.
If the game isn't in the list, it is most likely that you cannot give it away due to it being free in the past.

Which game did you want to give away?