Commented out the external_url for gitlab. Checking processes shows git 28439 0.6 17.3 1533024 355056 ? Ssl Oct19 7:32 sidekiq 4.1.4 gitlab-rails [0 of 25 busy] which is about 17% of my memory on this box.
Unicorn is up there in the memory usage, but the rest of the processes are reasonable and managable. I'm only running Mattermost for a test team of 6 users.
Let me know if you need further information or if you'd like a pubkey account access to this test box.