Had to look it up myself. I think it refers to Group Member Organizations, which would be the health insurance providers.

When you really think about it, health insurance companies are a bizarre sort of consumers’ union. Your insurance company negotiates prices with providers on your (and their own) behalf leveraging their buying power based on the size of the group. That was probably a good thing at one time, but now the system is so completely broken that if you try to get the same procedure done without insurance, and it’ll cost you double or triple what it would cost the insurance company.