We have a height gage on a granite block, and the gage needs calibration but the block does not.
This falls into Jim's answer above ("It depends")...
Our height gage is a single point gage measuring thickness of a flexible material...and the gage never moves in relation to the block...it always reads against the same point on the block...there fore it doesn't matter if the block is flat. There fore it does not need to be calibrated.
There are quite a number of "height gages"...which type is yours?
If the foot of the stand rides on the block, AND the gage is not directly over the foot, AND the measurement will be used to make decisions of import, AND the degree of flatness of the block can impact the reading to a significant degree compared to the needs...then yes, it would need to be calibrated.