Thanks for using and contributing to GitLab!
Although Zulip is far ahead of other open source solutions we are leaning towards Rocket Chat. Javascript and Meteor are the most popular language and framework so it would be easier to contribute. It is not who is ahead now but about speed of development.
Zulip is very heavy, they recommend 4GB for a production installation. This is due to the 15 workers that are not multi-threaded (GitLab has multi-threaded background workers).
We're proud that people can run GitLab on a Raspberry Pi 2. Forcing people installing the whole GitLab suite (GitLab itself and chat) to have a 8GB server is not acceptable. We're still hoping for multi-threaded unicorn foreground processes in GitLab so we can run it on 512MB.
By the way, Rocket Chat has 'native' clients on Android and IOS, although I think they use webview inside them.