Without knowing too much specifically of the gs motor, I am very familiar with BMW engines and it is not uncommon for the pistons to be the first to go and the cylinders and rest of the engine to look fine. Unlike most other engine the cylinder walls in BMW's are nikasil coated or treated. The makes them the metal equivalent to diamond. Pistons wear and the cylinder remains. Nice in a lot of ways since you don't have to overbite when you rebuild. New piston, new rings and a go. It is a little odd for the pistons to wear but something has to give no its not the wall, then.......
Good luck. Pictures would be great :)