After you receive the code for the alpha you have to activate it on your GOG profile, after that it shows up as another game on your shelf:
The installer is around 60 MB and after installing takes around 105MB of disk space.
After initial login the interface shows up and the program itself looks… boring to be honest:
And this is how installing a game from your library looks like:
You can see the Settings window open on the screenshot above. You can select the number of concurrent connections which speeds up the download speed I believe (no idea if it’s P2P?), the Startup Page which is something similar to Steam (either Library or Store), Features (not yet available) and installation paths.
I will post more stuff when the app gets updated. Right now it’s nothing special but it works.