You can get GOG GALAXY installation file from here. To download the installer, click on the purple DOWNLOAD GOG GALAXY 2.0 button. Once the download is complete, run the installation program, and follow instructions displayed on screen to finish the installation. Then simply launch GOG Galaxy, log in to your GOG account and enjoy!
Note that to log in to GOG GALAXY, you should use the same credentials you use to log in to our website.