some time ago,for 3 weeks in a row my players who get 60 - 65 minutes all stayed at lvl7 GS. to maintain lvl8 GS usually is not hard, but to get lvl9 it is hard enough. thats why i say there are big random thing there.
once i got 65 minutes for 2 players with different stamina - both stayed lvl8
i remember when my player with some 74 or a bit more minutes got GS from lvl8 to lvl9, although he played heavy minutes earlier weeks.
if random would not be big, then we would have some kind of chart or something, like it is with training, but we dont have one, we just know that usually about 63-69 minutes player have best chances to get from lvl8 to lvl9 GS.