Head shims depend on what % nitro fuel you are running and what heat plug etc.
The standard shims for the GX are 1 x alloy (.3mm) shim and 2 x brass (.1mm) shims, for a total of .5mm
Matt runs .3mm, I run .4mm, so everyone has their own combination of shims, fuel and plug that works for them.
If your new to nitro motors and tuning I would run the standard .5mm as comes from the factory, on 25% fuel with a med to hot plug. This will make your tuning a little easier to start with. With reduced shims, it can be a little harder to get a really nice tune and a steady idle unless your up with play in the tuning dept. Running standard shimming will make it easier for you to enjoy your motor, and give you more than enough power - trust me