When you retire at age 65 and start receiving Medicare insurance, there are a few things you need to be aware of – for example, supplemental insurance policies that are commonly known as Medigap. When you have Medicare Plans both A and B, you are eligible for Medigap. The reason you should definitely look into Medigap is because there are definite gaps in the coverage that you get from Medicare, and it can be extremely expensive to pay for all the gaps on your own.