Create an XMPP account specifically for testing. It will be easier to test things if you will use a small XMPP account without all your friends. Also, you won't annoy them ;-)
Create a different profile with that account. Use gajim -p test or ./launch -p test and set up all things.
Start Gajim with debug logging python3 -W default ./launch.py -p test -l gajim=DEBUG
If you want to contribute code (fix a bug, add a feature, ...) you have to create a Fork first (see fork and merge workflow). After adding your changes to your fork, you can create a Merge Request to the upstream repository.