Having just replaced my WP with the same GMB unit, I would GUESS that your mechanic does not realize that the objective is to replace only 1/2 of the water pump, not the entire unit! Even after all 8 bolts have been removed, you have to make careful use of a chisel or "cheap screwdriver" to break the seal between the two halves of the pump - after many thousands of miles that gasket has REALLY taken a set and is practically like glue. Once the gasket seal has been separated, the two pieces will come apart and it will be obvious to your mechanic where to go from there.
Good Luck,
Jab
