EDIT: I had another look at the GOG Client and realized that this is an upcoming feature. My bad!

BUT! For now I just created a .bat file to run Steam and Uplay with GOG Galaxy simultaneously. I also right-clicked my Steam and Uplay shortcuts and added -silent to the Steam shortcut target box and -uplay_silent to the Uplay shortcut target box, so GOG Galaxy starts maximized and the other two, minimized. You have link the shortcuts if you want minimized because the launch commands apply to shortcuts, not the exe.

Here are the .bat file commands

@ECHO OFF

start "" "<path to GOG shortcut here>"
start "" "<path to 1st launcher shortcut (not exe) here"
start "" "<path to 2nd launcher shortcut (not exe) here"
(etc.)

exit