Just some additional information about Medicare enrollments for Part B, D, Medigap etc.:
Regarding ACA/ Obamacare. If you only have Part A coverage, it is considered that you have minimum essential coverage. This means you can not enroll in any ACA/ Obamacare plans to cover any gap period until your Part B covers you.
Medigap plan enrollment window. To avoid medical underwriting where an insurer can’t deny you coverage due to pre-existing conditions etc., you have a 6 month window once Part B start date is effective. For a special enrollment situation, such as a US citizen returning to the US, once you have landed and got your US residential address to provide the insurance carrier, then the 6 month special enrollment window starts.
For Part D drug coverage, once you land back in the US you can sign up for Part D with an US address: Your chance to join lasts for 2 full months after the month you move back to the U.S.
Below is helpful link form Medicare enrollment windows and special situations:
https://www.medicare.gov/Pubs/pdf/11219-understanding-medicare-part-c-d.pdf