If you don't already know it, Jonathan M. Hethey has recently published a book about GitLab on packtpub. I am in the process of reviewing it, so wait for a new post very soon :)

I'll attach the preface so you can get a glimpse of what the book covers.

In this book, we will take a tour of the version control system GitLab, which is based on Git. It's open source and has many great features that we will learn chapter wise as listed here. First, we will take a look at what GitLab can do for us, and later we will see how we can install it on a server and configure it to match our needs. The next step is to take a close look at the web interface, and then we will learn how to handle permissions and teams, document our code, track issues, and show example workflows. Lastly, we will learn how to perform maintenance for our installation, including the creation of backups and upgrading to the most recent version. We will also cover getting in touch with the community and developers on the respective channels.

Btw, there is a limited offer on packtpub untill this Friday 3rd January, so you can grab an e-book copy for only $5.