How long you have to make a claim and collect on your GAP insurance policy will depend upon the terms of your policy. If your vehicle has been declared a total loss by your primary insurer then you should inform your GAP insurance carrier.
After the primary insurance company has determined the actual cash value of your car then your GAP insurance company should determine what amount they are to pay out on your policy. If you wait to make the claim it may cause take longer for you to collect on this policy and may need to explain why you did not inform them of the total loss when it occurred.
To find out for certain how long you have to collect on your GAP insurance policy read through your policy, talk with your GAP insurance carrier and even perhaps check with your state's insurance regulator to see what state insurance laws are regarding this.
Comment Update: If you don't have any insurance coverage, GAP will not pay any amount.