Change gem source in Gemfile

Use github repo for specific gem in Gemfile

Instead of

gem 'rails'

You can specify a github user/repo combination with

gem 'rails', github: 'rails/rails'

Make use of ref, branch, git for specific gem in gemfile

 gem 'any gem',git: 'any repo',branch: 'specific branch of that repo',ref: 'reference no.'

ref specifies individual commit. branch specifies the git branch to pull from.