Any way to sort hidden games by date?

No chance because no sorting function. Forget the game.

Option 2.35 and you can sort by add date in ESGST.

Do you remember how long? You can go here: https://www.steamgifts.com/account/settings/giveaways/filters load all the pages in only one, then "control+F" and look for "minutes" if you hide it, for example 50 minutes ago. So you can find it.

With Endless Scrolling and Table Sorter it should be possible to do that but it's a pretty bad way (you need to load all pages). Would be nice to have a native option as it has been requested several times already (e.g. here, here or here) but unfortunately has not been implemented yet.