Have you tried rebooting already? I remember it helping sometimes. If that does not help, check the value of $XDG_DATA_DIRS to be sure that it includes paths to flatpak directories. But when I tried flatpak before, I do not remember configuring that manually in Guix System.There may also be software with similar functionality in guix to what you are trying to use with flatpak, but I am unsure which may suit your needs.
I did reboot and it did not fix the issue. The packages are actually available through the nonguix channel, but I felt it's probably is safer to use the more widely adopted package on Flathub. Maybe I'll go for the nonguix packages instead.
I also found some Guix packages that extends GNOME. I'll try them and some other out-of-band extensions first.
(Sorry if mentioning nonguix is inappropriate.) Thanks!
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