I have several games installed, and I can start them through their .exe files, but they don't show up the Installed "tab" in GOG, nor do I see an obvious way to "show" GOG where they are.

Click the "+" at the top (between "Filter" and "Search"), then "Scan folders for GOG Games" then select the folder you want to scan.

Thanks. It worked. It's weird that Galaxy "lost" them in the first place.