Have a look at the gypsum industry website. It will explain the spec you have been given. It can consist of metal stud framing, one layer of 19mm core board and two layers of 15mm fire shield board each side with insulation in the middle. The spec could have changed to a different board layout. I would
Check out glenveagh(builder) than what is on the planning document.
One thing is Your neighbour could cut a hole in the plasterboard with a pad saw and rob your house while you were away. At least with a brick wall it will take time. Same applies to fire and sound.
Really the quality depends on what the sub contractor is being paid. Some ask for gypsum spec but it does not mean you actually get the materials and installation to gypsum spec.