Hey everyone! 
I am currently developing a native iOS client for GitLab, called gitlapp. Last year, I was on the outlook for an iOS client for GitLab since I got frustrated using the web interface on my smartphone. Alas, existing apps for iOS either suffered from the same issue or lacked essential features.
Hence, I started to work on an iOS client myself and will launch its beta soon. Most importantly, I want to gather feedback for gitlapp with this beta. If you are in need of an iOS client for GitLab, give gitlapp a try when the beta launches. 
While the app is not open source, you can always make your voice heard in the public issue tracker or ping me on Twitter.