You’d have to install it. Jekyll gets installed by your Gemfile which we automagically process using bundler; some other things (e.g. Gatsby) get installed by package.json or yarn.lock (using npm or yarn).
So - a good first starting spot is to wrap up your generator in an npm or ruby package that you can install as bundle install foo or npm install foo (those names are probably taken so you’ll need to be a bit more creative due to single global namespace) and then you can require them via those package managers’ config files, let us install automatically, and then we’ll be able to run your build command of foo 