Go into GIMP. Grab your Diffuse (or base color). Go into the layer tab and select mask. Make it black (full transparency). Then put your height map into a different gimp file (just right mouse click your file, then elect open with GIMP and it should open in a new window). Then select your entire Height map image. Copy it, and then past it into your Diffuse File. Your Diffuse file will then have transparency where your height map was. If you export this as a TGA, your color channels will remain the same, but your alpha channel will have your height map. Do the same with your normal and roughness maps.