[GNC] GnuCash Flatpak on GalliumOS Linux

[Art Chimes]

Anyone have any experience with GnuCash installed via Flatpak on
GalliumOS? (It's a niche distro designed to turn a Chromebook into a
Linux machine.)

I installed GC 4.1 and it runs fine ... except that it doesn't
recognize the USB drive where I my data file lives. The USB drive and
the data file, which I copied over from a working GnuCash installation
on a Windows machine, show up fine in Thunar (Linux) file manager and
via terminal CLI. The "open file" dialog format looks like GnuCash's,
not like something I've seen on my Gallium machine.

If I copy the data file to a location that shows up in the open-file
dialog (e.g. Desktop) it loads fine.

Any ideas how I can remove the USB drive's cloak of invisibility from
the open-file dialog?

Thanks, Art

PS -  I also posted my query in the GalliumOS subreddit, in case
someone there has a solution.