The heat transference is from a non-vented fireplace, 38,000 BTU. The sheet rock is 1/2 inch in size and on the opposite side of a 4 inch thick wall.
Sheetrock should not spontaneously combust from those levels of temperature. I’m sure there is a temperature that could cause most materials to be destroyed, but it wouldn’t be the levels experienced from a residential fireplace. Your fireplace should come with a manual that details the guidelines for installation to make sure that you have proper clearances. For instance, most fireplaces would not allow sheetrock to be in direct contact with the metal box. But common installation has sheetrock on the other side of the wall behind the unit.