Modern Medigap plans do not include drug coverage. Some of the older plans, those issued prior to 2006, may include a limited drug plan.
If you have a Medigap plan you should consider a stand alone drug plan. The beauty of this approach is you can find a drug plan that meets your needs. When you buy a Medicare Advantage plan with built in drug coverage you take the plan they want you to have.
Medigap plans purchased after January 1, 2006, are not allowed to have drug coverage. To get prescription drug coverage along with a medigap plan, you must purchase Medicare Part D. Medigap policies cover things not covered by Part A and Part B, like co-payments, deductibles, and health care if you travel outside the US.
If you have a Medigap plan you should consider a stand alone drug plan. The beauty of this approach is you can find a drug plan that meets your needs. When you buy a Medicare Advantage plan with built in drug coverage you take the plan they want you to have.