So...Geralt did all of that because he wanted the scars to be removed? I understand that they were magical and that they couldn't be removed by anyone else. But really, we don't see what effects the scars had on him.
He acts as if he had to follow orders but really, it's just a scar. Am I missing anything? Do we know that the scar removal was so important that Geralt was held hostage to do the main antagonist's bidding?
What made you think it was just a scar? From the first encounter, Gaunter told Geralt that it was a mark to show that they were associates, which tells us that those are some specific markings. If you go to Yen, she'll tell you that the symbols are ancient and powerful magic.
If we merely assume that Gaunter was a sorcerer from the get-go:
1. Then he's a sorcerer who can teleport without the use of portals
2. He doesn't need to actually wave his arms around unlike other mages to cast spells/use magic.
3. Because of 1. & 2. would he not then be considered among the highest if not the highest tier sorcerer?
4. The more power a sorcerer uses, the more exhausted the sorcerer becomes. Geralt could not ignore the pain when Gaunter branded him, which didn't even exhaust Gaunter in the slightest. Therefore Geralt can assume that Gaunter is capable of a lot more than just that. However, assuming the unlikely scenario that Gaunter was only capable of branding, then it also stands to reason that Gaunter could keep on branding his entire body, continuously causing Geralt pain till Gaunter got exhausted. It's a big risk though.
5. Gaunter freed Geralt by conjuring a storm instead of a key or breaking open the lock. When talking to him near the village of Yantra, he'll tell you that the more conventional means were too simple for him.
6. If Geralt asks him if he's a mage and if he cast a spell to conjure a storm, he'll reply that he cannot stand spells and that he finds them to be "childish hocus-pocus." He could be bluffing, but then there's all the other things to take into account, which would still make him very dangerous.