We certainly aren't in "normal" times and your problem, hopefully short-lived, isn't an easy one to rectify.
Your main problem seems to be the prophecy of the future and the timing of the matter.
In your case, because you have had interium coverage from your employer and you are older than 65 now, signing up for Part B will take time and documentation from your employer. You won't have a late signup penalty and you do have time (8-months) but the real problem is your insurance coverage for these Part B (and maybe Part D) coverage item.
Of course Part B can be stopped after it has started, but the process is filled with paperwork from you and your employer and is not an easy task even in "normal" times.
You can voluntarily terminate your Medicare Part B (medical insurance). However, since this is a serious decision, you may need to have a personal interview. A Social Security representative will help you complete Form CMS 1763. To find out more about how to terminate Medicare Part B or to schedule a personal interview, contact us at 1-800-772-1213 (TTY: 1-800-325-0778) between Monday through Friday from 8:00 AM to 5:30 PM. You can also contact your nearest Social Security office.
So signing up for Part B (and perhaps Part D) and then terminating Part B (and perhaps Part D) when your employer coverage begin again is possible but a pain and will require work and precision in timing. Then of course, you also mentioned a Medigap (GAP) policy too.
I find it odd that your employer is not keeping the coverage for just a few weeks more but guess that is up to them unless they have gotten a PPP loan/grant - where they should keep it going to qualify.
What has your employer advise you and any others in this special situation to do?
Think you will just have to make the best decision you can based on what you think the future is going to be with your job and employer coverage.