FIG. 1 is a front elevational view of the siding gauge according to our design FIG. 2 is a rear elevational view thereof; FIG. 3 is a top plan elevational view thereof; FIG. 4 is a bottom plan elevational view thereof; FIG. 5 is a left end elevational view thereof; and, FIG. 6 is a right end elevational view thereof. The broken lines are representations of holes and/or screw heads and form no part of the claimed design.