There's already a penalty for non-participation in a GAC round, you get zero rewards.
I agree with the league GP cap.
GAC is wholly broken right now. I just faced all 9M+ GP accounts in Carbonite. And they attacked, minimum amount to claim the win. All three rounds I faced people with more than twice my GP, who should probably be near Kyber. One had every GL except Leia. This is completely messed up. This has become "normal" in Carbonite. I have started skipping GACs, or only playing once so I can get minimum rewards. But it's basically pointless to participate at this point.
They should promote anyone in Carbonite over 3M GP. If they're really that bad at playing, they'll drop back down. But I'm sick of these 10M GP accounts that are living in Carbonite and preventing others from progressing.