Homebrew is nice. However unlike brew and npm, gem does not make aliases in /usr/local/bin automatically.
Solution
I went for a very simple approach (as of March 2020):
# Based on "`brew --prefix ruby`/bin"
export PATH=/usr/local/opt/ruby/bin:$PATH
# Based on "`gem environment gemdir`/bin"
export PATH=/usr/local/lib/ruby/gems/3.0.0/bin:$PATH
Add this to your .bashrc (or .bash_profile, .zshrc, etc.).
That's it! Now all Ruby bins and installed gems will be available from your shell!
In older versions of Homebrew (before 2017), there was a separate package for Ruby 2 called ruby20, for which you'd use the following snippet instead:
export PATH=/usr/local/opt/ruby20/bin:$PATH
This line was the only line needed at the time. But, in Ruby 2.1 the gems got moved to a separate directory. No longer under /usr/local/opt/ruby/bin, but instead at /usr/local/lib/ruby/gems/2.0.0/bin (where "2.0.0" is the last major Ruby version for Gem's purposes).
How it works
Homebrew keeps track of where it installed a package, and maintains a symbolic link for you that points there.
$ brew --prefix ruby
/usr/local/opt/ruby
$ l /usr/local/opt/ruby
/usr/local/opt/ruby@ -> ../Cellar/ruby/2.5.3_1
Effectively, adding /usr/local/opt/ruby to PATH is the same as the following:
export PATH=/usr/local/Cellar/ruby/2.5.3_1/bin:$PATH
Except, this long version hardcodes the currently installed version of Ruby and would stop working next time you upgrade Ruby.
As for Gem, the following command will tell you the exact directory Gem adds new packages to:
$ gem environment gemdir
/usr/local/lib/ruby/gems/2.7.0
Tools
These tools were meant to automatically bridge between Homebrew and Gem:
I haven't used these but they might work for you.
