I came across the attached article. In the introductory parts it briefly explains how air crew selection changed during WWII. May be of interest. On the Glider Pilot Regiment website it reads "The RAF aircrew selection system was used" for selecting GPs so I assume it was the same for those applying for transfers to the GPR. Perhaps someone can confirm or refute this. View attachment EVOLUTION OF APTITUDE TESTING IN THE RAF.pdf
I can't confirm the GPR aspect, but the document is correct from my experiences in the early to mid 1970s.