Hi Leighton, Jeff,
So long as the files are monthly and have the NetCDF attribute calendar_flexible set to 1 then they will be able to work in Gregorian calendar runs. You can also edit the metadata in the file to allow this using ncatted, e.g.
ncatted -O -a calendar_flexible,time,o,l,1 CH4_biomass_low_2014_2020_time_series.nc
Note that this would overwrite the original file.
You can use ncdump to look at the headers and work out which files, if any, have the incorrect setting.
Best wishes,
Luke