There are two problems here. Firstly, rvm is normally only loaded by the user's shell and that shell isn't being invoked here. Secondly, bundle won't be in the PATH either. To fix both problems, assuming this is a per-user rvm installation, try this...
... --exec /home/gitlab/.rvm/bin/rvm -- default do bundle exec ...
As a side note, you shouldn't use unicorn_rails with Rails 3 applications. Just use plain unicorn instead.