Speaking as another long time mh/nmh user, it would be nice to have a gmail/nmh/3rd party glue configuration guide in one place. Something along the lines of:
For gmail, you want to set it up for POP3 access and these other settings, which is found here (as of 2019)
For nmh, a sample set of config files (we are pretty close here).
As best I can tell, you do need some 3rd party stuff (fetchmail?) to make the two talk. Add that to the mix, with the disclaimers that the nmh folks are providing it as a worked at the time of publishing example.
If we get something that works, I'll volunteer to polish the wording (I do proofreading from time to time). I just lack the knowledge on implementation.
gmail does some things acceptably, but I have some use models where nmh is the correct tool for the job.