The problem is that the gift checkbox option is there in the "Buy Now" dialogue box for games, but does not exist in the "Download Now" dialogue box for games.
An example would be Kentucky Route Zero which I already have in my library. If I click on the "Buy Now" button for that game, I see the gift checkbox option near the payment type. For Mindustry's "Download Now" button, however, there is no gift checkbox there.
Thanks for the help!