IBM medical plans are NOT medigap plans. They do not conform to the federal government definition of a medigap plan. In fact, you can buy a medigap plan and also have IBM medical plan coverage. Why do that? Not for the medical coverage but to get the prescription drug coverage - which may be a better option for some people than the standard Medicare D drug plans. It just happens the medical coverage is part of the IBM package if you want the cheaper IBM drug plan.
It is unlikely you will ever get any benefit from the medical portion of the IBM plan if you have gap insurance too because you make the gap plan secondary and coordinate the gap insurance with Medicare. You'll never hit the IBM out of pocket maximum. Maybe if you have something done like acuputure which is not covered by Medicare and not covered by your medigap then the IBM medical will be useful. If so, file a claim for the service directly with United Healthcare. The form is on their website.
The Aetna Integration plans are not gap insurance plans either. They covers a lot of the same things that a medigap will cover but do not conform to the federal definition. It would make no sense to take an Aetna Integration option and then buy a separate medigap plan and a Medicare D drug plan.
The IBM hotline people often don't know this and will tell you that it is a conflict and you cannot have both IBM Medical coverage and medigap coverage. Medigap insurers don't know it either because they don't know the details of the IBM plan.