A GP is a test of stamina and mental concentration. It should be there to split the current best. Perhaps AA is 'lost' if a shooter takes time out or misses a series. Make AA hard to get, easy to lose.
If a GP is only for a small elite then there may well be serious cost issues?? Why would a club host a GP if relatively few can participate (1 or 2??)? Unless they charge for it as a means to support other events for 'all grades' ?
So, if you can get Lottery funding, for example, then restrict a GP to the very best otherwise it would be based on simple charity from non-AAs - time costs money, fuel, etc. After all it is a lousy spectator sport :-)