My GOG Galaxy client auto-starts whenever I start Windows. Unchecking the "auto-start" feature in the client settings does nothing. Do I have to uninstall Galaxy to keep it from auto-starting?

If you deselect the option in: Settings -> General -> 'Launch GOG Galaxy at system startup'... and the client still starts once Windows starts, then that's definitely a bug! There's no other hidden option afaik. I think it works fine for me, but haven't checked since the last patches. You should probably report it through the 'report issue', including the 'logs' folder.

works fine for me mate

I got the same issue, and this sucks.
Uninstalling Gog Galaxy right now.

press ctrl+shift+esc to open the task manager. go to "startup", then look for gog galaxy. right click and pick disable

Get Autoruns: https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns

(and go pale seeing all the clutter that starts with Windows without ever asking...).

But be sure what you're doing if you use this. You can actually brick your system if you deactivate vital services.