It's important that the following information considers Michelin and specifically the Pilot Road 4 and the Road 5. So is the GT for across the board "heavy bikes" or bikes being fitted with loaded panniers or the fact that the motorbike is rode with two?
According to Michelin, this is a misconception when it comes to their products at least. The fitment "GT" has nothing to do with the weight of the bike. It is simply to do with the handling of the motorcycle where Michelin feel a tyre with a slightly different casing construction would be beneficial.
At high speeds using a standard tyre can cause high speed wobble, which is why Michelin recommend the GT tyre is fitted, ensuring the rider gets the best feel and ride he would expect in all conditions.
Below is a list of motorcycles that Michelin recommend the GT version: