Tiff with signed samples

Hi,

I'm trying to process this image, a 2G tiff with apparently signed samples:
https://astrogeology.usgs.gov/search/map/Mars/GlobalSurveyor/MOLA/Mars_MGS_MOLA_DEM_mosaic_global_463m 


If I try to "gm convert" to a smaller size so I can actually edit on my 
computer, it converts, but the samples seem to be read as unsigned, 
making very light and very dark areas.

I can mess with the pixel values to get something that seems right, but 
is there some option I'm missing to convert properly with gm?

-Tom