Try opening the settings and set your folders so they are matching those in the Downloader, if they aren’t already. It just automatically picked up a game I downloaded and installed specifically for this reason, so it would seem that was the only reason preventing it from picking up installed games.
With TW3 releasing in less than 3 weeks it’s wise to focus on getting the basics right, it’d seem logical to have a wider release to fish out any remaining bugs.
Also a launcher, but anyone’s already using Steam, could simply keep adding their GOG games in the Steam client and do without an additional program.
On the upside, it’s very smooth looking, underlines how outdated Steam’s industrial styling is, cons: I’m still a fan of Steam’s dark theme.