Short answer: Medicare Advantage (Part C) replaces Original Medicare with a private plan that has networks and often extra benefits, usually at a low premium. Medigap works alongside Original Medicare to pay its out-of-pocket costs and lets you see any Medicare provider, but costs more in premium.
They’re two different ways to fill the gaps in Original Medicare. A Medicare Advantage plan bundles Parts A, B, and usually D into one private plan, typically low premium, network-based (HMO/PPO), with copays and extras like dental or vision, but you’re limited to the plan’s network and rules. A Medigap (Medicare Supplement) policy instead pairs with Original Medicare and pays much of its deductibles and coinsurance, letting you see any provider that accepts Medicare nationwide, but you pay a higher monthly premium and buy a separate Part D drug plan. Advantage trades flexibility for lower cost; Medigap trades higher cost for freedom and predictability. Switching between them later can require medical underwriting, so the initial choice matters.