chibizoid: Hi. As the topic clearly states, GOG Galaxy 2.0 Beta cannot be stopped from launching whenever I start my computer up.
Permissions to allow GOG Galaxy to launch at login have been all deselected from every possible preferences window available.

DOCK
I have made sure the Open at Login feature is deselected (directly from the Dock).
MACOS SYSTEM PREFERENCES
The System Preferences / Users & Groups / Login Items pane does not even list GOG Galaxy in it. So there is nothing there to remove.
GOG GALAXY 2.0 PREFERENCES
I oppened GOG Galaxy and from its own Preferences I unchecked the Login at Startup option.

And yet, without exception, every time I turn on my computer there it is GOG Galaxy launching without permission.

I include a screencap of my computer at the moment where GOG Galaxy is trying to load and how its Dock icon shows that the option to open at login is unchecked. And I inserted the preferences pane of my Mac to show how it is not included in the items that are actually allowed to do so. I didn't include a screencap of the Galaxy's Preferences pane because I already uninstalled Galaxy, but trust me, I made triple sure the option was unselected.

And since I'm here...
ANOTHER BOTHER
That is not the only problem with it that I experienced, lethargy is another.
GOG Galaxy takes around 10 seconds just to close when I quit it. Even when I'm already logged-off.
In general, not only it starts without permission, it slows down the whole startup process of my computer considerably.

My computer's specs are as follow:
iMac (21.5 inch, late 2012)
OS: MacOS Sierra (10.12.6) (16G2136)
Processor: 2.7 GHz Intel Core i5
Memory: 8 GB 1600 MHz DDR3
Graphics: NVIDIA GeForce GT 640M 512 MB

That's it.
I've tried thrice now to use GOG Galaxy and love it, alas, those lofty goals have never been reached.

I hope this helps depurate the software and make it better.
:)