Medigap is coverage for things that Medicare does not cover. You will find that there are limits and deductibles with Medicare. Medigap will do away with all of you out of pocket expenses or most of them depending on your plan. I strong suggest that you look into Medicare Advantage. Medicare pays insurance firms per person and in return for using them, the insurance company provides better benefits than regular Medicare does. For instance, for free I get a $6,000 limit on my out of pocket. However I pay $80 a month to limit my out of pocket to $2000 year plus I get some other benefits for my money.
Best thing you can do is to go to the AAPR website and search for Medicare plans. Then call them and you will find them very helpful, They will ask you questions, even what medications you are taking, then suggest a plan for you or more likely a choice of several plans and explain the differences. You also need to get a plan that is supported locally. Not all doctors will accept all the plans, some may not even accept Medicare as we found out.
Also remember to file for it on time or else it will cost you more later. This is too complex a topic go be answered in a post so talk to AARP and they will set you straight.