You don’t have write permissions for the /Library/Ruby/Gems/2.3.0 directory. (mac user)

below is what I need to do.

To run the specs, you’ll need to install RSpec. First, run gem install bundler in the root directory of your project. Then, run bundle install. To run a single spec file, run a command like this: bundle exec rspec spec/00_hello_spec.rb. To run all of the specs at once, run bundle exec rspec.

So, I typed gem install bundler in Terminal, and got the error:

You don’t have write permissions for the /Library/Ruby/Gems/2.3.0 directory.

and this was in the project file in atom

source "https://rubygems.org"
gem "rspec", "~> 3.2.0"

My question is:

It seems like terminal is giving me the response because I’m not supposed to change anything on ruby, and I need to bundle install inside of atom? Could anyone tell me how to use atom or run anything in atom?