Either you were given a load of bs or you misunderstood what was said.
The Intake manifold (and gaskets) is NOT on the bottom of the engine.
You can check to see if the radiator is plugged (Yourself) by simply removing the rad cap (when the engine is COLD or cool) and idle the engine while watching the coolant in the rad. If it circulates evenly and with good volume it should be good.
You would be wise to get another opinion. Try to find a reputable independent shop for your repairs and not a dealer chain. Ask friends or co-workers who they like.
Does the engine overheat?
It IS possible the IMG is leaking though. (One of GM's finer boners over the years)
Have you noticed the coolant level dropping in the coolant reservoir?
Check for light tan colored oil on the bottom side of the oil filler cap.