A gusset preheating assembly and a method of using the same. The assembly includes a heating gun assembly mounted on a frame. Spreader rods open up a gap in a bag's gusset and nozzles blow warmed air into the gap. Orthogonally arranged first and second hydraulic members are operatively engaged with heating gun assembly and are activated to move the heating gun assembly vertically or horizontally. Hot air blowing into the gap from the nozzle relaxes the gusset. Pressure is applied to the relaxed gusset, thereby reducing the thickness of the gusset region of the bag. The reduction in thickness makes it possible to increase the number of gusseted bags that are packaged in a container or on a roll.