Can't force quit application no matter what

I have never seen this behaviour in the 14 years I've been using a mac: I'm working with Gimp (image editor) and it suddenly freezes (showing the spinning beach ball).

That's not so uncommon, but what IS uncommon is that there is literally no way to quit or force quit the app except from rebooting.

This is what I tried so far:

• trying Force Quit: the only effect that this has is that the application eventually disappears from the list in the Force Quit dialog. But it's still visible, and beach-balling happily along;
• opening the activity monitor: nope, that seems to be frozen as well;
• going to the command line:

$ ps -ax | grep gimp
1583 ??        38:13.74 /Applications/GIMP-2.10.app/Contents/MacOS/gimp
$ kill 1583

• nothing seems to happen, so ...

$ ps -ax | grep gimp
1583 ??        38:13.74 /Applications/GIMP-2.10.app/Contents/MacOS/gimp
$ kill -9 1583

• still nothing ...

$ ps -ax | grep gimp
1583 ??        38:13.74 /Applications/GIMP-2.10.app/Contents/MacOS/gimp
$ sudo kill -9 1583
Password:

• still nothing.

What's odd is that the application doesn't show up in either the Force Quit dialog, and the little indicator light underneath it in the dock is off.

I tried clicking on the Gimp icon there, and it started to bounce as if starting up normally - only, it kept just bouncing without opening.

There's nothing showing up in the Console app either when I filter on "gimp".

This is now the second time this happens; the first time was about two weeks ago.

Any ideas what's going on here?

Mac OS Catalina 10.15.4

16'' MBP late 2019 2.3 GHz, 8-Core Intel Core i9, 32 GB 2667 MHz DDR4

AMD Radeon Pro 5500M 8 GB / Intel UHD Graphics 630 1536 MB