The C4 model for visualising software architecture Context, Containers, Components and Code diagramming methodology is an alternative to UML, this particular example taken from here.
The C4 model is an "abstraction-first" approach to diagramming software architecture, based upon abstractions that reflect how software architects and developers think about and build software. The small set of abstractions and diagram types makes the C4 model easy to learn and use.
You can create C4 diagrams in GitUML by using the PlantUML macros from https://github.com/RicardoNiepel/C4-PlantUML by simply including
at the top of your "Refine UML" area. Also, since at the moment you must base your diagram on a repository and select at least one file in that repository, you will need to select some random repository and file and hide its contents with hide statements in your Refine UML area e.g.
This limitation (of having to select a repo and at least one file) may be lifted in the future, allowing pure PlantUML diagrams to be hosted. Potentially some integration with the classes and modules in the repository and C4 techniques may also be developed - its early days in my thinking yet.
List of repository modules/files being visualised in the above diagram: