Does anyone know if there are any statistics on the percentage of gaming PCs that are prebuit and how many are custom made?

Do you mean among GOG users, or in general?

And as a lot of shops do over prebuilt ones, which you can still modify (by the shop, like other HDD, GPU....), how do you count those? :P

EDIT: And just to cut if a bit short, NO, there is no statistics, just think of all the small computer shops they are not in any statistic anyway ;)