Regarding your specific question, since you can't carry inside your car without a CPL, you'd have to get out, arm up, pump gas, disarm, then get back in your car. Is that what you're asking?
According to the Michigan Liquor Control Commission, parking lots are not considered part of the premises, just the structures are. However, I don't know if a gas station pumping pad would be considered a parking lot similar to Meijers. Clearly it is not a structure, either.