With ERM you can give a hero skills only temporary, the amount of skills is not limited, so in this case, the skills are given to GM-Adventurer at advanced level only for the duration of the fight. After the fight, they get removed. If the Hero has one of these skills already at expert this does not happen ofc.
But I recently found, it can happen that if you level up after combat, one of these skills can become permanent. I already made a fix for it, but it is not yet uploaded.

Overall I'am not 100% happy with this last perk for GM-Adventurer but until now I could not come up with any better solution. The idea is that if you manage to reach GM-Adv it means you most likely had to renounce on skills like Armorer and Offense. So you are rewarded with a little extra punch, in the end, to keep up. But while playing I found that the GM-Adv is already strong, even without these skills because he can accumulate so many creatures during the later stages of the game.