To use PostLab you’ll need a mac running Mac OS X 10.11 El Capitan or higher.
PostLab also requires to have Xcode Command Line Tools installed. If you haven’t installed this PostLab will install it for you. The Xcode Command Line Tools are needed to install Apple’s version of Git. PostLab will also install Git LFS. This is sometimes needed if a large file ends up in your library.
PostLab obviously needs Final Cut Pro and works with version 10.3.4 until the latest one.
If you want to use the PostLab interface you’ll need a GitLab Server running at least 10.3.