On a Mac, all you should have to do is
sudo gem install jekyll
Then get the bundler (like npm, but for Ruby)
sudo gem install bundler
Quit terminal, reopen it. Go to the directory that contains the Gemfile and install all the things.
cd src
bundle install
You may be prompted for your admin password.
The root of the directory has a package.json file to make your life easier.
To build and start a local preview with livereload, go back to the root directory and run
npm run start
Open a browser to localhost:4000.