At first, place the figures on the canvas and enlarge it to desired size. Then select Filters->Artistic->Random pattern texture...
Make sure you've installed the pip package manager for your GIMP's Python 2 installation (GIMP on Linux usually uses systemwide Python 2).
Then run python -m pip install -r requirements.txt to install plugin's dependencies.
Finally, place genpattern directory from this repo into your GIMP's plug-ins directory.