You can sign up for a Medigap plan once you are enrolled in Medicare Part A and Part B. You must have both parts in place to qualify with MOST carriers.
There are a few Medigap carriers that will issue a policy if you only have Part A but they will pay claims as if you have Part B in place.
There are a few Medigap carriers that will issue a policy if you only have Part A but they will pay claims as if you have Part B in place.