Shikabashi Sorry I didn't communicate this well. I suspect it's the same problem, because once the Range option is displayed correctly, the GIF export is also successful. I sent you a video via email to clarify.
I know this is weird, but the Range option will show up even if you just open a different Spine project once and check the GIF export settings. In the video example I sent you, I opened the Spineboy project once, opened the export window, and then re-opened your Spine project. This resolves the Range option not appearing in the export settings, and when I uncheck the Range option and run the export, there is still a long calculation wait before the export starts, but the export results are correct. The GIF files are exported for each skin and they are animated correctly.
