Here are a list of featured diagrams, showing the kind of things you can do with GitUML.
These are diagrams from a sample Python GitHub repository which has been set up for testing GitUML's Python parsing capabilities. You can select this sample repository via the new diagram wizard if you want to get started quickly and just have something to play with.
The source code in the repository is not a coherent application, but rather various modules and classes which exercise the GitUML Python parser, including a mix of Python 2 and Python 3 files to ensure GitUML can handle both flavours of Python.
These are diagrams from the GitHub repository of one of my other projects, Pynsource, a desktop UML app for Python.
This project has been going for 10+ years and is open source. It actually led to the creation of GitUML and shares parts of its Python parser.