When you have regular Medicare you are able to purchase and use a Medigap policy of your choice. However, if you select a Medicare Advantage plan, similar to a HMO policy, you no longer qualify for a Medigap policy. If you continue paying on such a policy you are literally throwing your money away. Once you are enrolled into a Medicare Advantage plan there will be no reason for you to have a Medigap plan. Medigap insurance policies are policies that cover the expenses that Medicare does not handle.
If you have discontinued your Medicare Advantage plan you have guaranteed issue rights for at least 63 days after the policy has ended. Guaranteed issue rights mean that you can apply for a Medigap policy without prejudice by the insurer. After those 63 days have expired you will be subject to any pre-existing condition clauses or other underwriting requirements the insurer may have. During this 63 day time frame you are eligible to qualify just as if it were open enrollment.
Medigap policies are only designed for use with regular Medicare. You cannot use these policies for any other type of coverage. You must pay additional money for an Advantage plan. By purchasing gap insurance needlessly will just be a larger expense for you. If you are concerned about deductibles and co-payments with your Advantage plan it is wise to shop around for your plan. While all plans are required to provide the exact same service, they are not required to charge the same. By finding a value priced policy you will have more available cash to use toward co-payments.