FIG. 1 is an upper perspective view of the spray gun showing my design. FIG. 2 is a front view thereof; FIG. 3 is a back view thereof; FIG. 4 is a side view thereof; FIG. 5 is an opposite side view thereof; FIG. 6 is a top plan view thereof; FIG. 7 is a bottom view thereof; and, FIG. 8 is a side elevation view thereof, showing an attachment in broken lines. The broken lines are presented for illustrative purposes only and form no part of the claimed design.