A Medicare Supplement (Medigap) insurance, offered by private companies, covers healthcare costs, which Original Medicare is unable to cover – coinsurance, co-payments, & deductibles.
If you are covered by Original Medicare, and you purchase a Medigap plan, Medicare will cover its share of the healthcare costs. The Medigap policy will do the same.
A Medigap policy is not the same as a Medicare Advantage Plan. Medicare Advantage Plan serves to get access to Medicare benefits. A Medigap policy, on the other hand, serves only as an addition to the Original Medicare.