The BMW M6 was a car developed and designed by the motorsport division of BMW to be a high-performance car that can handle driving long distances. The Gran Coupe version simply means that the car is no longer a "standard coupe" but has four doors to make getting in and out a bit easier.
Shopping for quality parts for your BMW M6 Gran Coupe when it's time to make repairs or perform maintenance starts with shopping OEM. Original equipment manufacturer parts should be your only choice when you're looking. This is because they're made for your M6 Gran Coupe specifically and are tested in the factory to ensure they're top quality. Aftermarket parts may be cheaper, but they also can come with a whole host of issues that can cost you a lot more in the long run. They can be made with subpar materials or require more work to "make fit." You don't want to have to make something fit in your vehicle - you should be able to make a purchase and be confident that it will work in your car. You don't know if an aftermarket part will be made with the same standards - so shop OEM so you can know.