I am recently retired. I still have health insurance (GHI) I also have Medicare. Which should be my Primary?

The reason i ask is because GHI is rejecting all claims stating that Medicare is my primary. Do I have a choice in who is my primary? Which is better to have as my primary health insurance. Thank you.
--------------------
I think your primary should be Medicare and is better since GHI is rejecting your claims. But if you don't know, you should go ask them and find out. Hoped this helped, and I'm sorry I didn't really know how to answer this at all. But best wishes, and lots of luck(:
Source