Short answer: No. You cannot use both at once, and it is illegal for an insurer to sell you Medigap while you are in a Medicare Advantage plan unless you are switching back to Original Medicare.
When you start Medicare you generally pick one path: stay with Original Medicare and optionally add a Medigap policy to help with out-of-pocket costs, or enroll in a Medicare Advantage (Part C) plan that bundles your coverage, often with extra benefits. A Medigap policy exists to fill gaps in Original Medicare, so it does nothing for someone whose coverage instead runs through a Medicare Advantage plan.
If you have a Medicare Advantage plan and want a Medigap policy, you first need to plan a return to Original Medicare. You can apply for the Medigap policy before your Advantage coverage ends, but the Medigap coverage cannot start until the Advantage plan ends. Remember that buying Medigap outside your six-month Medigap Open Enrollment Period may require medical underwriting, so timing matters.