No, it's not real. The open enrollment period for Medicare supplemental insurance (which runs from October 15 to December 7) does not extend to Medigap supplemental insurance.
Medigap is a form of private insurance that pays for out-of-pocket costs not covered by Original Medicare. (If you're under 65 and have Medicare because of a disability, see the next question in this section; the rules are different for you.)
Your insurer should give you a monthly statement outlining the overall out-of-pocket expenses for covered prescription medications, as well as whether that number puts you in or brings you out of the coverage gap.