While I'm not a true MIM or GRS expert, I'm reasonably certain both MIM and GRS provide for multiple system data set ENQ. I haven't kept up well with GRS, but I think GRS uses SYSPLEX communication facilities. If you haven't defined this capability, then you want to think real hard about continuing to pay CA Technologies, or whatever their name is this week, their outlandish license fee for MIM. Further, MIM has a number of other capabilities. You should check if you're paying for and using these additional capabilities. If you're not using them, you should discuss this with CA and get them to remove them from the MIM license, which should save you some money. This may back fire on you. CA used to bundle a lot of stuff into a package and sell you the package at a discount. Take stuff out of the package and you'll pay full price on the rest of the stuff.
I strongly recommend you read MVS Planning: Global Resource Serialization for your z/OS release before you proceed further.