Greetings! There was a topic from back in February, where people where having problems installing GitLab on ARM. The reason was that a specific gem, therubyracer doesn't build for ARM architectures. The solution is to remove it and its dependencies and replace it with nodejs. You can find the modified PKGBUILD in my github repo or the gitlab mirror.
For the record, it took nearly 100 minutes to build on a Raspberrypi B. If anyone is interested, I'm thinking to setup some machines for Cross compiling and use Distributed Compiling in order to provide a repo. Then we could discuss adding it to the ARM aur one