I use Geometry Generator a lot in QGIS. I promised in a tweet to make some of my use cases available online, therefore I made a repository with my examples on GitLab. A great introduction to QGIS Geometry Generator by Klas Karlsson can be found on Youtube: https://www.youtube.com/watch?v=0YxjJ-9zIJ0.
In the repository on GitLab you can find some Example Results, the QML-files and Sample Data to try the examples yourself.
Dimensions for Polygons
Details of this Geometry Generator Style
Dimensions for Lines
Details of this Geometry Generator Style
Direction with one arrow per line of a polyline
Details of this Geometry Generator Style
Labels with a leader
Details of this Geometry Generator Style