Medicare Prescription Drug Gap Disclosure Act - Amends title XVIII (Medicare) of the Social Security Act to require that a prescription drug plan or a Medicare Advantage Prescription Drug Plan with an initial coverage limit obtain a signed certification by an enrollee (or legal guardian), before enrolling, that acknowledges that: (1) a gap in coverage during a given year might occur; and (2) the enrollee, in such a case, will be responsible for paying 100% of the cost of his or her prescription drugs, in addition to the plan's monthly premium.