So I found the downloader on the website and intalled it.
I tried to launch the game but it displays a pop-up error saying :
Setup file 'gameinfo.txt' doesn't exist in subdirectory 'C:\Program Files (x86)\Steam\SteamApps\sourcemods'.
Check your -game parameter or VCONFIG setting.
I have no idea what to do with that, if somebody could lighten this up for me
Thanks in advance !