Billy Bob
That is exactly what I did as well. The hardest part of that job was actually driving the hour to the drywall wholesaler to buy it.
My suggestion to anyone out there getting ready to do or redo their hearths is to do your research and don't short cut the R-Value requirements by the manufacturer or the inspector if one is involved. Also don't forget to properly seal the edges where/if the hearth butts up against a wall or existing firebox.