It might be Webers gullex B as it was 0-4mm in ballast at that time, now they have changed it to 0-3mm. But gullex is a simpler mortar and might not be the most suitable for plastering a basement. It was probably easier for the contractor to use the same for both masonry and plastering. Buy plaster mortar B and apply and brush it, first, you need to clean the damages from loose plaster...
