gem install gocardless complains about the oauth2 version with a clean install. The specified dependency is ~> 0.5.0.rc1 but the latest is 0.7.1. Easily fixed with gem install oauth2 --version '0.5.1' or resolved using bundler, but just thought you'd like to be aware if you're not already as the docs suggest starting with a gem install.
gem install gocardless
gem install oauth2 --version '0.5.1'
You'll have to excuse me if this is expected behaviour. I'm still not that familiar with the ruby ecosystem. :)
Update oauth2 dep version, fixes #16
I've updated the oauth dependency to v0.7.1, which appears to fix the issue. The updated gocardless gem is on rubygems as v1.0.1.
Thanks for raising this!