I am unclear where you are, I am in the US. Your provider should be informed before delivery (they typically require the VIN, year, color) and you can present that question. Gap insurance is likely a formality and they should automatically include it. They do this daily, they understand the gravity of this situation, and it is their job to cover that major risk. If you already listed the vehicle with them I wonder if they itemized the gap insurance and you are merely unaware of it. There should not be a reason to purchase gap from a different insurer unless you hate your provider and they already ground you down to peanuts on pricing.
If this is a loan, gap may be included in the loan as the bank wants to make sure they are covered, so there is something for the dealer or bank to show you on the paperwork.
