What kind of error do you get trying to use GPMC? If you are still an administrator on the domain, then you should be able to undo any policy settings that are causing problems, manually. The key is to understand what you are locked out from. Did you get get removed from the Domain Admins group by this policy? If not, then it should prevent you from undoing the policy. If you are just being restricted from using the GPMC MMC snap-in then can you still run AD Users and Computers? If so, then un-install GPMC and use the old AD U&C method of unlinking that GPO from the domain.
In the future, before linking a GPO, make sure you use the Group Policy Modeling feature in GPMC to analyze what settings *will* be applied rather than finding out the hard way 🙂