I don't know any addons or plugins in order to import GMT files into Blender.
The most usual way is to open the GMT with 3DSimed and export it as .3ds
Blender can import .3ds of course.
3Dsimed can import and export GMT files ( and export in .3ds or other possible formats )
Some 3D edits are possible in 3DSimed.... but limited compared to Blender or Max
Once GMT exported to .3ds file, you can import and edit in Blender and re-export in .3ds which you'll have to edit in 3DSimed in order to re-export ( after some needed operations ) to .gmt file.