Hi guys, Arch Linux + GNOME user here. (I already posted this in another topic, but I think it will be useful here too.)
I was finally able to solve this issue now! I already had sni-qt installed, which wasn’t making the icon show up, however I noticed someone saying about conflicting indicators apps, so I went on and uninstalled libappindicator-gtk3 and libindicator-gtk3 - which I had installed a long time ago - and the icon started showing up after starting the session.
I don’t know exactly what’s happening, since I wasn’t able to inspect any logs that might lead to the clues, but my guts tell me insync might be trying to use libindicator-gtk3’s API first instead of sni-qt and end up “thinking” that it’s already showing up. In other words, libindicator-gtk3 might be tricking insync that it’s showing up properly, when it’s actually not.