You can clone with
HTTPS or Subversion.
I couldn't get the gollum tab in a project to work without changing the Gemfile to specify a v2.2.0 tag:
gem "gollum", :git => 'git://github.com/github/gollum.git', :tag => 'v2.2.0'
Otherwise, it would error-out with uninitialized gollum constants (only when viewing that project tab).
The git master version was used because there wasn't released version at that time. Now we can use the published version.
This fork is a bit rotten. There a fresh forks out there: https://github.com/gugod/redmine-gollum/network