To my knowledge, you can get ground resonance from two scenarios - one being an out of balance rotor system (seized or faulty drag damper or similar) and the other being when the blade passing frequency excites the natural frequency of the undercarriage. You can of course have the two combined.
I have experienced GR due to uneven tyre pressures on a Wessex during rotor start which therefore affected the natural frequency of the undercarriage and I know of a couple of occasions when a drag damper lock wasn't removed before start up leading to a very unpleasant rotor start.
Normally the designer goes to some lengths to avoid any natural frequencies of the rotor system stimulating those of the undercarriage but I presume the same effort isn't made for RC models.