The recent EES and Bronze editions of Gitlab have a great feature for automating mirroring source projects. If your project depends on open source projects that are not under your control, mirroring is a wise choice. Full documentation is available at Gitlab Repository Mirroring
The mirroring feature is not available in the free plan, upgrade to the bronze plan – $48 per year – is required. I could spend some time writing my own mirroring software using git (set remotes, fetch all, pushing and monitoring the processes), but getting this right and monitoring it, would take probably more of my spare time than I would value $48, and also.. Gitlab is great software, why not just support it!
This screendump shows how to create a new project from an existing git url:
If you do not have a paid subscription, the project settings -> repository will show that you need to upgrade to get access to the repository mirroring feature: