Step 1 :
Use the wget command followed by the URL of .deb file of GitHub desktop from the shiftkey Github repo. Shifykey is maintained by Brendan Forster one of the staff member of Github.
sudo wget https://github.com/shiftkey/desktop/releases/download/release-2.6.3-linux1/GitHubDesktop-linux-2.6.3-linux1.deb
Step 2 :
Install gdebi using the following command which helps you to install local deb packages, resolving and installing its dependencies.
sudo apt-get install gdebi-core
Step 3 :
After installing gdebi, using gdebi install the GitHubDesktop client using the following command.
sudo gdebi GitHubDesktop-linux-2.6.3-linux1.deb