Let's say you want to make something available as a gem, but it's just too trivial for the rubygems.org repository. GitHub Gists can be accessed via git, so perhaps you could host a Rubygem there? You can!
This gist is an example of a gem I've called 'tinygem'. It has no functionality, but defines a module called Tinygem.
You can use it from any Gemfile like so:
gem 'tinygem', git: 'https://gist.github.com/6077471dd4783c4b476384930d3dc60a.git'
And then the Tinygem
module will exist within your app.