Newer gitlab versions do provide pagination support via a header.

See https://github.com/gitlabhq/gitlabhq/commit/0678b8a4 Gitlab v6.8.0+
Add a new method that follows the "next" rel if present in the link

header, and use this to fetch all repositories.

Tested on a gitlab 6.8.X deployment against a developer reviewboard.  Was able to add a repository that's beyond the first 100 repos.