This has been an issue for quite a few versions now: The system tray icon for Gajim doesn't show. Reinstalling or fiddling with the relevant settings doesn't help either. I suspect the tray icon file is missing.
Maybe it's related to this issue:
https://forum.artixlinux.org/index.php/topic,8709.0.html
I should've waited before making the topic lol. I forgot to mention useful info. I use Gajim package 2.3.5-1. The system tray is managed by Trayer, specifically the AUR package trayer-srg-git r130.b5693f5-1, which I noticed hasn't been updated in 4 years... Although even the trayer package from the extra repo is not working either. Other packages like Nheko, Element, SuperScreenRecorder, all show tray icons just fine. Maybe it's an issue with GTK4 applications specifically?
Edit: I've tried switching to gdk-pixbuf2-noglycin from the omniverse repo, to no avail.