wget https://gmaster.io/Installer fails.
GMaster-Installer.exe which is hard-coded to download http://s3.eu-west-2.amazonaws.com/gitmaster-releases/releases/GMaster-0.9.580.zip
For off-line installation, you can download this
zip file, then uncompress it and go from there.
Note that after that, it still needs access to https://gmaster.io/api/session/login, so real off-line use is impossible.