I have a game for giveaway but I can't find in the site selection, how can i proceed??

You can't giveaway free games. If the game isn't in the list, then it was free/exploited.

the game has been excluded from being given away on this site, because it has been given away for free in the past in large quantities.

Which game?
As mentioned above some games have been removed. If you have a very new game, it just might not be in the system yet.

Edit: Welcome to Steamgift!

I understand... I won 3 games and neither of them was in the system... I think it's because of what Oppenh4imer said.

Thanks for the help

Select Support - Create a new ticket and select Category from drop box Add Game to List

yea they are giving away like 500,000 keys for that game today for free and i believe its not the first time its been a free game, so you will not be able to give that game away here:) you might want to read the FAQ section of the site at the top of the page before making or entering any giveaways:)
Murder miners free