I think that there's no other tool than this one ( Geditor ) ... but there's a solution to have it working also for GMTs:
Open 1st a texture .gtr file, then from the menu load the one of the Objects .... at opening and/or once you click on a GMT, click OK for the error message and there you go ... the export and import are perfectly working for GMTs.
If you wish to export more than 1 GMT from the same .gtr just click on the 1st one you wish to export and use CTRL_click ( if separate )or Shift_down arrow ( if more than 1 ) for the next ones.
That way you won't have to export them separately and have to click OK for each one you should select.