Two weeks ago, GitHub released support for Mermaid diagrams in files and most Markdown fields (see Include diagrams in your Markdown files with Mermaid). However, gists were missing support for Mermaid. Now, gists do support Mermaid diagrams just like discussions, issue comments, pull request comments, and files on GitHub. Still to come, and very important, is wiki support for Mermaid diagrams.

For more information about using Mermaid with GitHub, see Creating diagrams in the GitHub documentation.