So you either have to use Galaxy or wait until the next public build.The only other place to get it as far as I am aware is Steam. Sorry but if you want features like this you have to use Galaxy, and there really isn't any reason not to.
If I wanted to install some 3rd party gaming platform on my PC and turn it into a pseudo-console, then yes, I would just use Steam instead of GOG. But many of us are here precisely because we aren't interested in 3rd party installs.
It is a reasonable feature request to include experiment builds in the Library through a similar opt-in feature where you can access the builds and take responsibility for their installation.
I contacted support to see if they could provide a direct link to whatever experimental builds are being offered on Galaxy and they denied, twice, that there are any on Galaxy. Which versions are you seeing there?