Why would your 98 GMC Jimmy blow hot air even when the temperature gauge is set to cold?

Because the Main Gear in the Blend Motor has cracked and is now not turning the heat/cold door to the Cold position. 3 fixes: Buy a new Blend motor, takes about 5mins to replace. Buy a new Blend Motor Gear, will take about 30mins to fix. Repair current gear with Epoxy, takes about 30mins and 12hrs for the glue to set up strong enough to use it.