Using my netCDF viewer (HDF Explorer) it tells me that the data type of the gdalwarp .nc file is NC_BYTE, and this is a 1 byte signed var
The other issue. GMT recognizes global grid registration grids with first col at -180 and last at +180 as well as the case where the last col is at +180-x_inc. First case, and this is from memory, even allows the case where first and last column are different (when they shouldn’t be), case in which, I think, both columns are averaged. But this is the GMT flexibility trying to recover from 3rd party liberties