You don't need to file anything to the inside of the stock, it's just the outside of the stock where it comes in contact with the receiver
File from the inside and it will be invisible. If using an aftermarket trigger housing you need to do additional filing of the stock at the top to get the stock pin to align properly (needs to slide in 3mm more).
Put the stock on the GSG and you will see at the top where you need to file.