valensaz93: Why??? Is there any other way to install a game apart from using galaxy??
Yes, there should be three alternative ways to install any game from GOG, all essentially boiling down to simply downloading the stand-alone installer.
The most basic, and probably also most cumbersome way is to go into your account's library on the website by clicking "ACCOUNT", followed by selecting the game you want, which will open a menu with all the available download options. Just download each file listed under "GAME DOWNLOADS" into the same directory; and once it's done, double click on the setup .exe in said directory to start the installation.
The problem with this method is that the download of these large files through your browser is prone to data corruption (particularly if your internet connection isn't entirely reliable), resulting in the installations not working.
Alternatively, Galaxy has an option to do this for you. By opening the game's entry in Galaxy's library view and selecting "MORE", you can find the option "Extra downloads". In there should be a section labelled "GAME BACKUP COPY"; click on it and Galaxy will download what are essentially the same files as described in the first method to its designated "downloads" folder.
Of course, I don't know how well this function currently works.
I generally find that the best way to download larger games without issues is still the old "GOG Downloader". It's actually supposed to be replaced by Galaxy, but the latter's functionality is still rather shaky from what I hear.
To make use of it,
download the application's installer here; install and open it. Afterwards, open your library (on the website, again), select the game and click on "MORE", then "GOG DOWNLOADER LINKS". Use the link that's appropriate to your system and the game will get queued up in the downloader. I think the default download destination is "documents/GOG.com Downloads", so that's where you should be able to find the files.
Additional note: There seems to be a severe lack of appropriate synonyms for the word "download" in the English language.