"I don’t see how this suggestion would ever create a scenario where an active account is in a tough luck situation."
It absolutely would. I would say I'm above average/well above average power/ability for my GP. I started in A3 and climbed to just hitting K3. However, since then I've fallen and climbed, fallen and climbed. Hitting a low of A3 (once) again and peak of K4 (twice)
Those well below average ability for the the GP are totally screwed by your system.
I really can't see why we can't deal with players that are falling because of inactivity and see where the land lies after that.
GP really isn't accurate in determining how well a player does in GAC. Especially the further they fall, and the less of their GP they can use.