This is a super simple example Rails app demonstrating how to use Railgun / mailgun-ruby with a Rails app.
I recommend using asdf-vm to build a Ruby install. Once you've set up your Ruby install:
# Make sure you've got Bundler installed
gem install bundler
# Install the Rails app bundle
bundle install
# Configure your Mailgun API key and domain
vim config/environments/development.rb
...
# Change the `default from:` address in the mailer
vim app/mailers/default_mailer.rb
# Run the Rails server
bundle exec bin/rails server
After running the server, you can navigate to http://localhost:3000/users/new and sign up. You should receive an email shortly after finishing your sign up.