After some digging around in the Dolphin/Gamecube SDK files I had, I’ve discovered that there was a version of the “preview” program (for viewing .gpl 3D models) for Windows this entire time!
I feel like an idiot for somehow not noticing this sooner.
The app launches just fine. Unfortunately, I got a version-mismatch error when trying to load any actual files (including the examples that came with the SDK).
With that said, the source code for the program included with the SDK. My plan of action right now is to try removing the version-check code, then recompile the program.
If this works, we might finally have a way of viewing 3D models from the game, without me resorting to trying an old 90’s-era Macintosh.