When your first eligible buy a policy. The greatest time for you to purchase your Medigap policy is during Medigap open enrollment period which last 6 months. So anytime during the 6 – month enrollment period.
You can purchase any policy in your state Medigap offers even with preexisting health problems. The enrollment period will automatically start the same month you turn 65 years old and have enrolled in Medicare Part B Medical Insurance. You cannot buy a Medigap policy after this enrollment period. If you can it will most likely cost more.
Then during open enrollment:
Generally, Medigap insurance companies; to determine if they will accept your application; are allowed to use medical underwriting to calculate what to charge for your Medigap policy. Yet you may buy any Medigap policy a company sells if you turn in your application during your Medigap open enrollment period. Same price for both people in good health or if have some medical problems.