cant pair phone with car after forget this device. help

If you "forgot" the car on the iPhone, then go into your car settings and remove the phone. Then put the two devices back into discovery/pairing mode. This is done on the phone by just turning Bluetooth on. You do this however the vehicle instructions say, but generally there is a button and/or command somewhere in the radio system to pair a mobile device. Then follow the onscreen instructions.

I suggest that you remove the vehicle from the phone's list, and the phone from the vehicle's list. Turn Bluetooth off on the phone and power both the phone and the vehicle off. Then turn both devices back on, and put the vehicle into the pairing/discovery mode as indicated in the vehicle's user manual. Then go into Settings on the phone and turn Bluetooth back on. The devices should discover each other, and you should be able to follow the onscreen instructions to complete the pairing. If you continue to have difficulties, stop by your GMC dealer, and they should be able to assist you with paring issues with the vehicle.

The last thing I can recommend, before going to see your dealer, is to reset your network settings. Understand that doing so will also remove other Bluetooth devices and your wi-fi passwords. Go to Settings>General>Reset>Reset Network Settings. If that still does not help, then contact your dealer.

That doesn't make sense. "Forget" is not permanent. Your iPhone should recognize your car, not the opposite. Go to Settings > Bluetooth. You should see it as a device.

Is the car in pairing mode?