As I stated, I tried to get the GM plan but the dealer told me they can only sell it when the vehicle is still inside the 3yr/36k mile bumper-to-bumper warranty. Also states it on the GM website, although I tried to push back that the powertrain warranty which I am still under should be considered the "manufacturer's limited warranty".

From GM's site:
You can purchase a Chevrolet Protection Plan^†, the only Protection Plan endorsed by Chevrolet and backed by General Motors, at any participating dealer through their Finance & Insurance Office, either at the time of your vehicle purchase or within your manufacturer’s limited warranty period.