Although I own the vehicle; I am not the main user on the main vehicle insurance policy; in whose name do I apply for GAP?

If you bought the vehicle and are the main user on your vehicle insurance then it would be you and you would be the policyholder.

The policyholder must be named in the GAP Policy Schedule, the one whose name is the registered keeper of the vehicle in respect of a leased vehicle and the authorised user of the vehicle (if applicable) who is named in the Finance Agreement – all of whom should be mentioned in the main vehicle insurance policy.