I think Mr. Lappos is being kind to transmission designers with his estimate of 2-1/2% losses within the MRGB of a small turbine helo. With some light turbine helos the main gearbox efficiency situation gets a bit complicated.
Consider the example of the OH-58D with the 650shp M250-C30 engine. The engine has a speed reduction gearbox that drops the power turbine speed from >30krpm down to ~6krpm at the main gearbox input. So the main gearbox only needs to provide a speed reduction ratio of ~17.4:1. The small input/output reduction ratio and low input speed allows good efficiency from this main gearbox design, and under optimum operating conditions the 97.5% efficiency number provided by NickLappos would be achievable. But since some of the speed reduction is performed by the engine's gearbox, it does not accurately reflect the full drivetrain losses between the power turbine and main rotor. If all the speed reduction was performed within the main gearbox, the peak gearbox efficiency would be something like 96%.
Another important factor to consider with main rotor gearboxes mated to turboshaft engines with high speed power turbines, is the effect of speed/load on gearbox efficiency numbers. There are various types of losses produced by the gears and bearings, such as friction, viscous, windage, etc. Friction losses tend to be more load dependent, while viscous and windage losses tend to be more speed dependent. I've attached a graph that illustrates just how rapidly gearbox "efficiency" drops off under conditions of constant speed and reduced torque. This is a great illustration of the effect of speed dependent losses on gearbox efficiency, and why you will usually obtain best gearbox efficiency at max load for a given speed.