Thanks for the video!

The pauses when changing animation and clicking Export is because Spine must do many calculations to determine the number of frames and other information. Your project must be quite complex or very large resolution for the pause to be so long. For most projects it isn't noticeable. We have plans to improve it, so the UI shows a wait indicator instead of freezing. The processing needs to be done though, it cannot be avoided.

That the frame range doesn't appear seems to be a bug. Is it possible to provide your .spine file? That may be enough for us to see it, without needing images. We can't fix a problem without first seeing it ourselves.

Shikabashi Thank you for sending us your Spine project! I have confirmed that I can reproduce the issue in Spine 4.2.08 and have created an issue ticket here to fix this issue: EsotericSoftware/spine-editor790

Shikabashi Sorry I didn't mention that. I think these problems are related. Because reducing the complexity of the skeleton to some extent, where the Range option becomes visible, also solves the problem of slow response when performing the GIF export.

Also, when the Range option becomes visible, you can see that it is set to "0 to 0". This leads me to suspect that the complexity of the skeleton has exceeded the limit of something that can do a GIF export, resulting in the Range being detected as 0 and the export failing.
I will add a comment to the issue ticket mentioning this.

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.

Shikabashi Warm up is available to obtain a smooth loop. Try set a value of 1 or greater:

This is the number of time to play each animation before export, allowing physics to start in motion. Note setting this will make the export take longer. If your animations are created to be long for smooth loops, it is better to shorten them once to the duration you actually need and then export with Warm up.