I thought once you deleted a Mastodon account that username became permanently unavailable?

Maybe?
But this person did it somehow, but then they're also the instance admin, so there's that, maybe.

Aha maybe they entirely reset the instance?

IDK how they're doing it, but on Mastodon I can just follow them back just fine, but apparrently not on gts.

I've had a couple of times on my instance where a user I am trying to follow is showing as me requesting to follow them. I've fixed it by unfollowing them on my side and then waiting for a while for whatever cleanup to happen then following them again. Have you tried following the user by going to their page and choosing the follow button and going through that process?

Yes, however that unfortunately doesn't work on gts when I tried it.