The gains come from tweaking the balance between quality & speed. The GFX settings you see in the game options aren't the only settings, there is stuff that can be tweaked outside of that...plus some of the game setting can simply be straight up overridden on a driver level regardless of what the game says.
So while the FPS increase you see is real, its not more FPS of the same thing.
Its a useful tool on slower PCs, but for high spec PCs I'd strongly recommend giving it a miss....mainly because of the services side...that stuff is a wild maze of cross-dependencies & scenario specific behaviour that even the good tweaking tools/guides is hit & miss. Last time gamebooster fck'd up something on my system I ended up re-installing...couldn't reverse the damage despite hours of trying.
