It simply stats to launch the notifier from the command line
once you have downloaded and extracted the source package, you don't have to compile anything: the notifier is a Python script ready to use. Just run it and you will be notified when a mail is received at your Gmail account. Of course you have to edit the notifier.conf file to specify information about your Gmail account and some other option. Then you may want to run the notifier automatically every time you log in: look at documentation of your desktop environment to see how to start applications upon login.
For example in GNOME you go through menu System --> Preferences --> Sessions and then add a Startup Program. Instructions for KDE are in the README file of the gmail notifier.