This is a GUI version of the old 2000s tool named TGCtoGCM.
.TGC is a proprietary file format created by Nintendo themselves, which is commonly found on GameCube demo discs. It is the only official "shrunken" ISO/GCM format from Nintendo.
Nerdy info:
TGC is basically the same as GCM, except shrunken in size from 1.4 Gigabytes to merely two-hundred/three-hundred or so Megabytes. It also parses metadata differently.