I'm running Gitlab 4.1 with more-or-less the default installation instructions and very light usage currently. I'll upgrade to 5.0 in the near future. Here's what I'm seeing:

    $ top
      PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
     1616 gitlab    20   0 1851m 108m 7112 S   0.0 10.9   7:46.35 ruby1.9.1
     1405 gitlab    20   0  967m  99m 7044 S   0.0 10.0   0:26.63 ruby1.9.1
     1667 gitlab    20   0  970m  98m 2596 S   0.0  9.9   0:01.36 ruby1.9.1
     1670 gitlab    20   0  970m  97m 2488 S   0.0  9.9   0:00.77 ruby1.9.1

    $ ps aux | grep [g]itlab
    USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
    gitlab    1405  0.0 10.0 990904 101508 ?       Sl   Mar19   0:26 unicorn_rails master -c /home/gitlab/gitlab/config/unicorn.rb -E production
    gitlab    1616  0.1 10.9 1896392 110880 ?      Sl   Mar19   7:46 sidekiq 2.6.4 gitlab [0 of 25 busy]
    gitlab    1667  0.0  9.9 994156 100412 ?       Sl   Mar19   0:01 unicorn_rails worker[0] -c /home/gitlab/gitlab/config/unicorn.rb -E production
    gitlab    1670  0.0  9.9 994204 100284 ?       Sl   Mar19   0:00 unicorn_rails worker[1] -c /home/gitlab/gitlab/config/unicorn.rb -E production

This doesn't include Redis or database.

It seems like that's far too high. Why do you need 400MB of runtime data (and 5GB of allocation) in order to generate these relatively simple web pages?

Ruby is bloated.