About FNP ?

I think you are fine to specialize on one thing. I suggest you reach out to or join a specific professional organization to find out about certification. The only "concern" about doing FNP is if you really want to do acute/critical care.

Check this link out: https://www.hopkinsmedicine.org/gastroenterology_hepatology/education_training/nurse_practitioner_fellowship_program.html

And this one:

https://gastro.org/aga-leadership/initiatives-and-programs/nurse-practitioner-and-physician-assistant-resource-center/

You can work in Internal Medicine (including Gastro, Cardio, Onco, Nephro and everything else) clinic with FNP without any problems or additional certifications anywhere in the country. You can also do specialty rounds in hospitals (if your job description and contract require this) in many (but not all) places and states.

There are specialty certifications available through corresponding specialty organizations but they all require regular (and at times quite high) fees, matter not that much overall and completely voluntary. Only one notable thing you can get through these organizations is specialty CME. Before COVID19, there were also networking options through conferences but now it is all placed on hold indefinitely.

You do not need specialty nursing experience for this, but try to get friendly with specialty consultants if you now work in a place where they appear.