vinochino: So this is an old thread but anyway.
Epic had Battlefront II for free a short while ago. While getting it over the Epic Launcher, I actually had to install it via Origin Launcher. GOG also discovered the Origin Game as installed, not as Epic game. However, after playing it for a little while I uninstalled the game via Origin (it wasn't even possible over Epic even though it was shown under installed games). But GOG won't discover the Game as uninstalled. It still appears after several days now. I also tested if GOG would update after opening all other launchers, nada. I even restarted the PC and opened the other launchers before GOG.
My guess, it cannot detect the uninstallation because of the Epic/Origin cooperation.
Got the same Problem with Mass Effect Andromeda ... bought it at Steam and it is linked to Origin Launcher
But GOG Galaxy shows the Origin Version as installed even thou it's uninstalled for weeks now.