I was trying to get journal entries for Xorg.bin, however, entering "Xorg" in the search field gives no results, not even if "all" is selected in the left pane. 

I guess gnome-logs should have some custom filtering capabilities, for starters the ability to find output from a specific command, the equivalent of

journalctl -e _COMM=[command]

would be useful.

Second, I would at least want to know, what to expect in the various categories in the left pane. What filters are these? 

A log viewer is targeting the more tech savy user anyway, and that would justify giving him/her some power to use the app in the way they want. For now, I always find myself using journalctl instead, because gnome-logs is so inflexible.