The default 2-D binary netCDF grdfile in GMT has several attributes. The grdedit utility program will allow you to edit parts of the header of an existing grdfile. The attributes listed in Table B.1 are contained within the header record in the order given (except the -array which is not part of the header structure, but makes up the rest of the file).
GMT also allows other formats to be read. In addition to the default netCDF format it can use binary floating points, short integers, bytes, and bits, as well as 8-bit Sun rasterfiles (colormap ignored). Additional formats may be used by supplying read/write functions and linking these with the GMT libraries. The source file gmt_customio.c has the information that programmers will need to augment GMT to read custom grdfiles. We anticipate that the number of pre-programmed formats will increase as enterprising users implement what they need.