Their offering is here: https://www.stackit.de/en/ You have to be a company to make business with them. You can not just sign up, you have to contact them first.
There was a discussion about STACKIT some years ago: https://news.ycombinator.com/item?id=30853778
I do not see anything about Gaia-X on their directly website, only when I search for it there are some older press releases.
This is not necessarily a bad strategy if they want to do something different than OVH/Scaleway/Hetzner.
For instance, I understand you cannot possibly run a decent mail infra on the above hosting vendors IP ranges because they are so popular cheap self service hosting service they have been used a lot by spammers.
So definitely not targetted at the lone developper starting a side gig but more as a vendor for the many larger EU institutions/companys that mostly kept their stuff on prem until now because of patriot act.
Menu -> Compute Engine -> Pricing