A. Don Jackson responds: No, you can't use gypsum board to reduce clearances from combustion appliances and vent pipes. You may be thinking of the use of drywall in firewalls and other fire-resistive assemblies, which are intended to slow the spread of fire, not to prevent initial combustion.
You'll need a heat shield to reduce clearances to a woodstove pipe. One of the most common is a piece of 24-gauge sheet metal, attached to the nearby wall or ceiling with standoffs so that there's a 1-inch air space behind it (see illustration, below). If properly installed, this can reduce an 18-inch single-wall pipe clearance to 9 inches...
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