Ruby keeps every gem version, and every Ruby you installed

Ruby's storage problem has two layers. Gems are kept at every version any project has ever needed, and if you use a version manager, that entire collection exists separately for every Ruby you have installed.

Measure both layers

du -sh ~/.rbenv ~/.rvm ~/.gem 2>/dev/null
gem env home
du -sh "$(gem env home)" 2>/dev/null

gem env home prints the gem directory for the Ruby currently active, which is the one you are about to clean. With a version manager there is one of these per installed Ruby, and the others are not affected by anything you run today.

The command that removes old versions

gem cleanup --dryrun
gem cleanup

gem cleanup removes old versions of installed gems, keeping the newest of each. Run the dry run first and read it, because a project pinned to an older version through a Gemfile.lock will need that version reinstalled afterwards. In a Bundler project that is one bundle install away, which is why this is usually safe rather than always safe.

The documentation nobody reads

Every gem installs its documentation by default, and on a machine with hundreds of gems that is a meaningful share of the folder. Nothing on a modern setup reads it locally.

du -sh "$(gem env home)/doc" 2>/dev/null
gem cleanup && rm -rf "$(gem env home)/doc"
echo 'gem: --no-document' >> ~/.gemrc   # stop it happening again

Rubies you no longer run

Each installed Ruby is a full build plus its own gems. List them and remove what nothing pins:

rbenv versions      # or: rvm list
cat ~/**/.ruby-version 2>/dev/null | sort -u

The second line prints every Ruby version pinned by a project on the machine. Anything installed that is not in that list is a candidate for removal, which is the same approach that works for the Node versions nvm keeps.