The correct answer is "It entirely depends on the receiver, and possibly the settings".
If it's important, I suggest testing it by flying at 5,000 ft directly over the top of a ground way-point and look at the distance being displayed.
If you want to be really anal, remember also that "ground distance" is a very imperfect term. Does it mean "surface distance"? Highly unlikely, so it will almost certainly only be an approximation, as Jim59 stated, based on a strategy like WGS84 (but not necessarily that one), which means that the "ground" distance being calculated could be an imaginary surface several hundred feet above or below the surface you're flying over - although unless you're flying a long way, that won't make a massive difference.