It’s worth a shot giving GM customer service a call since your dealer already turned you down. Be polite and professional, but explain that it’s barely over the warranty on miles and well within the warranty on time.
If that gets you no where, as a last resort you can try something along the lines of, “It’s unfortunate that you can’t find a way to resolve this. My family convinced me to purchase this car after talking about the great service they have always received on their GM vehicles. I’m going to be sure to inform them of your strict interpretation of your warranties and suggest they look elsewhere for a future vehicle.” Do not however word it along the lines of “This will be the last god damn shitty gm me or my family will ever buy”.