How to give away keys for "unlisted" games?

For games that are listed on Steam but don't show up on the giveaway creation page, you can create a Support ticket and select the option "Add game to list" from the drop down menu. For games that have gone F2P or are no longer listed on Steam, I would just dump the keys in a place where you expect people who might use them will frequent.

If the game is not in the list on SG it is normally given away free somewhere. If you don't wanna wait for it to come back to the list, you can drop the key in the Orphan keys drop thread but I prefer to post them in group/private GAs.

If a game is not listed, and it's not currently being given out for free somewhere, you can send a support ticket about it, and they'll add the game.

If game removed from steam its hard 😥
answer from support:

Game is not present in Steam database, so it will have to be added manually by the admin. He is aware of this issue, but I can give no estimate on when it's going to be resolved.