Yes, in most cases, you can cancel gap insurance in California. Legally, lenders in California must allow you to cancel a gap waiver at any time and refund any unused portion of the waiver. If you bought gap coverage through an insurance company, you can cancel at any time and receive a refund, typically.
Keep in mind that if you leased or financed your car, the lender may require you to carry some form of gap coverage. You might need to cancel your gap coverage if you sold your car or switched to a new gap coverage provider.