This is a little hard for me to diagnose, since the flag does show for me when I try to purchase and gift one of the games from my library.
The only workaround that comes to mind right now would be this: To the best of my knowledge you do not have to be logged in in order to purchase a game. If you log out before buying it as a gift you “might” be able to see the option again.
If it still does not show… Is there an option for developers to exempt their games from being giftable? Is that a thing? (That would seem weird, though…)