Galaxy 2 is supposed to be a platform for not only your GOG games but also others. They allow you to link games from other platforms (Steam, UPlay, etc.) and individual programs in though the method is not always explained well. Yes, Galaxy 2 does have problems like not obvious way to access their forums and games under other launchers still need to run those launchers to work.

Still if you have it, here is how to access Shadow Era under Galaxy 2, maybe the SE Devs would look into ways to make SE work with Galaxy features and attract Galaxy users unaware of SE.

1 - Under Games on left side, go to Owned Games, Installed or (Any of these 3 will work.)
2 - Click the + symbol up top.
3 - From menu choose Add Game Manually.
4 - Type Shadow Era in the box.
5 - With SE highlighted in the box, choose Link Executable. You can also click the game to go to its screen and from the game screen choose Link Executable up top.
6 - Navigate your PC folders to find the Shadow Era Launcher.EXE file, choose that.
7 - SE will now appear in your Owned and Installed lists, you can run the program in Galaxy 2.

I can provide screenshots of the steps if needed, here is how it looks in the Galaxy launcher.

GOG SE5.jpg

GOG SE6.jpg