Medicare Supplement Insurance, also called Medigap, helps fill the gaps that Original Medicare Part A and Part B don't cover. Think of Medigap as a helper that pays for things like co-payments, coinsurance, and deductibles. If you have Original Medicare, getting a Medigap plan can add more benefits. But, Medigap plans don't work with Medicare Advantage plans. You can only use them if you switch back to Original Medicare. Sometimes, your job or a union might help pay for extra insurance that Medigap doesn't cover.