Project Setup Advice

My company is looking to package common functionality into a plugin. This functionality includes, among other things, custom repositories for the buildScript and build process e.g. myCompany() and myCompanySnapshot() i.e. similar to How to create custom repository type?. However, since this needs to be shared across 100+ micro services, copy-pasta is not a viable solution.

I am not familiar with Gradle muti-projects, which might be a solution. Off the top of my head we could potentially use a git submodule to provide a plugin and use gradle to ensure that the submodule is updated properly. What is a maintainable method for including buildScript functionality as a shared plugin?