I dont know a whole lot about the GRP engines as I have never owned one. I think guys using the orange head version told me a 6mm insert worked best. I just checked and GRP offers them anywhere from 5-9mm by 0.5mm.
I would check with others who run that engine though. I have been pretty much strictly OS engines for the last 3 years but will be switching to the Reedy engine when it comes out this spring.