rbenv-clean
is a plugin for rbenv that aids in removing gems from your Ruby environment.
A "fork" of @jbernsie's work after he mysteriously disappeared from this planet.
$ mkdir -p ~/.rbenv/plugins # create the plugins dir if you haven't already
$ git clone https://github.com/meowsus/rbenv-clean ~/.rbenv/plugins/rbenv-clean
Uninstall all non-default gems in Ruby 2.5.1 and leave every other version un-touched
$ rbenv clean -v 2.5.1
Uninstall all non-default gems in the current global version
$ rbenv clean -c
Nuke the entire environment
$ rbenv clean -a
View help
$ rbenv clean -h