When reading GTM file in GRD format you actually read a NetCDF format.
using python and pylab to read it enable matplotlib to plot it.
say we have a grd file named z.grd which contains 3 variables: x,y and z.
reading z values in python is done by the following three lines:
from pylab import *
from scipy.io import netcdf_file as netcdf
data = netcdf('z.grd','r').variables['z'][::-1]
Now data is a numpy array containing z values in the file dimensions.
for more info look at: http://gfesuite.noaa.gov/developer/netCDFPythonInterface.html
