Given the following is broadly true:

• Rubies installed to ~/.rubies (via ruby-install most likely)
• Gems for each ruby version installed under ~/.gem (via chruby most likely)

What to do when you want to reclaim some disk space? Delete unused ruby versions of course! Pretty straight forward, look in ~/.rubies for ones you want to remove, then delete them.

$ ls ~/.rubies
ruby-2.0.0
ruby-2.1.7
ruby-2.3.1
ruby-2.4.1

$ rm -r ~/.rubies/ruby-{2.0.0,2.1.7}

Then the problem is we’re left with artifacts hanging around, namely any gems we installed for ruby 2.0.0 or 2.1.7 are still present under ~/.gem using up disk space. We could go through and find them by hand, or we could get the computer to delete anything under ~/.gem that doesn’t have a corresponding runtime under ~/.rubies.

diff --old-line-format= --unchanged-line-format= --new-line-format=$HOME/.gem/ruby/%L \
  <(ls ~/.rubies | sed -Ee 's/ruby-|-p[0-9]+//g') <(ls ~/.gem/ruby) \
  | xargs -pL1 rm -r

(xargs -pL1 will prompt with each command it wants to run before running it - answer y to proceed, anything else to prevent it running that command. Lets you see what ruby versions it is removing before it does so.)

$ ls ~/.gem/ruby
2.0.0
2.1.7
2.3.1
2.4.1

$ diff --old-line-format= --unchanged-line-format= --new-line-format=$HOME/.gem/ruby/%L \
  <(ls ~/.rubies | sed -Ee 's/ruby-|-p[0-9]+//g') <(ls ~/.gem/ruby) \
  | xargs -pL1 rm -r
rm -r /Users/caius/.gem/ruby/2.0.0?...y
rm -r /Users/caius/.gem/ruby/2.1.7?...y

$ ls ~/.gem/ruby
2.3.1
2.4.1

And now revel in your reclaimed disk space. (Hunting other large folders/items on your disk? ncdu is a great tool for that.)