Same problem here on Red Hat with gitlab-7.0.0_omnibus-1.el6.x86_64.rpm.
Error message reported in the log:
Started GET "/gary/libdatabroker/tree/master" for 128.250.146.86 at 2014-07-10 15:21:12 +1000
Processing by Projects::TreeController#show as HTML
Parameters: {"project_id"=>"gary/libdatabroker", "id"=>"master"}
Completed 500 Internal Server Error in 409ms
Adding RedCloth to the Gemfile fix the issue for me with gitlab 7.0.0.
The Gemfile seem to just be missing from the file unlike the other used by github markup gem.
By Administrator on 2014-07-14T17:00:16 (imported from GitLab project)
By Administrator on 2014-07-14T17:00:16 (imported from GitLab)