Or create an .app bundle so Gajim can be run from the desktop as a native app:
pip3 install PyInstallermac/makebundle.py
You can now go to ~/Applications and open the Gajim app.
Note: when using the .app method, the cloned gajim repository has to stay at it's original location ($(pwd) at time of creation), or Gajim.app will fail to run and you will have to re-run mac/makebundle.py.
If Python complains about unknown locale, try this (with your country code):