You can enter the GP's via the BFTA web site when booking opens ,its open to all grades.
However you might have to shoot the first 3 GP's as A grade until they establish your ability. This is done to protect C graders from throphy bandits ,who think that winning C grade when they shoot at A grade level is an achievement .
You have to shoot 5 out of the 7 to place and I think you have to shoot at least 3 to get in the end of season raffle.
About as much use at field target , as a life guard at the Olympic swimming pool.