Just to chime in with my experience. I hit 85 on December 24 and was placed into Carbonite V based on my GP at the time. My first nine matches in Carb V were largely against people in the 400-600k range and I won all of them and got promoted to Carb IV.
I'm guessing I was around 700k GP when I got promoted, I'm currently at 940k. The opponents I've faced are at 1.8M, 1.4M, 1.1M, 1.5M, 1.3M, 1.7M, 1.8M, 900k, and I'm currently facing a player at 3.1M. With one or two exceptions there's no strategy here--if they bothered to attack I lost.
I'm in the second lowest division. There's no way to just catch up to players GP-wise (other than spending, of course). If I can't win combats I get fewer credits. If I lose matches I get lower weekly payouts and I won't climb divisions so I'll get lower daily payouts. All of that is going to slow progression. It's looking like my GAC experience for the foreseeable future is going to be signing up, placing defenses, trying to win a combat or two, and hoping my opponent no-shows for some reason? That's not great.