How do I install ruby gems on Mac

brew update
brew install ruby

# If you use bash
echo 'export PATH=/usr/local/Cellar/ruby/[version]/bin:$PATH' >> ~/.bash_profile 

# If you use ZSH:
echo 'export PATH=/usr/local/Cellar/ruby/[version]/bin:$PATH' >> ~/.zprofile

# Note:
# Replace [version] with the install version for example: 2.4.1

You can do that but I suggest using an Environment Manager for Ruby.

You have rbenv and RVM. IMO go for rbenv:

brew install rbenv ruby-build

# bash
echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
echo 'eval "$(rbenv init -)"' >> ~/.bash_profile  

# zsh
echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.zprofile
echo 'eval "$(rbenv init -)"' >> ~/.zprofile  

# list all available versions:
rbenv install -l

# install a Ruby version:
rbenv install [version]

# set ruby version for a specific dir
rbenv local [version]

# set ruby version globally
rbenv global [version]

rbenv rehash
gem update --system

# Note:
# Replace [version] with the install version for example: 2.4.1

For more information check

https://stackoverflow.com/questions/39381360/how-do-i-install-ruby-gems-on-mac