I think Geis is decent at a lot of things but great at nothing aside from maybe long shots and free kicks. He's sort of physical but not really a ball winner, he's not painfully slow but not fast either. He can handle the ball but I don't think he has the technique or composure to outplay an opponents pressing or beat someone 1on1. He can play a good long ball but at the same time his passing is sloppy, he passes it high way too often and not precise enough for my liking and his short passes aren't always on the ground and crisp either.
I think he can be a good squad player, but I don't necessarily see the potential to be a top player at a club which plays CL every year in him (which is what Schalke surely aims for). He's had one decent season at Mainz, but I wouldn't say he carried them. Something close to €8-10m (without add ons) would've been a fair offer imho, but Heidel is a top DoF for a club like Mainz, he realized that Heldt was under immense pressure and that a couple of clubs with money were interested in Geis, so he bled them dry.
That's just my assessment though, maybe
@ctp can give us better judgement, since I'm sure he's seen the most of Geis.