You do not need GMmapTool to change the name of an .IMG map file.

If your Garmin supports different names of map files just rename them to what you like.

Example:

Etrex 22x:
I have openfietsmap (Germany).img renamed to TopoDE.img.

When I look on my Etrex 22x it shows TopoDE.img

Some older Garmins will only let you use these names:

gmapsupp.img (maps you download, can be both internal and external on a SD card ) Some of the newer Garmins also have names like this D6159220A.img

(this name is most common, and it means you can copy it into your GPS internal memory or memory card)

gmapprom.img (built in maps like City Navigator, only internal)

gmapprom1.img (built in extra maps, only internal. City Navigator EU and US need two files)

gmapbmap.img (base map file, can only be internal (on the GPS device)

gmapoem.img ( OEM maps, only internally)

So what Garmin are we talking about that you have?