Apart from the [official git documentation](http://git-scm.com/book/en/Git-and-Other-Systems-Migrating-to-Git) there is also
user created step by step guide for migrating from SVN to GitLab.
[Benjamin New](https://github.com/leftclickben) wrote [a guide that shows how to do a migration](https://gist.github.com/leftclickben/322b7a3042cbe97ed2af). Mirrors can be found [here](https://gitlab.com/snippets/2168) and [here](https://gist.github.com/maxlazio/f1b593b0d00aa966e9ca).