I claim the ornamental design for the depth gauge, as shown and described.
FIG. 1 is a perspective view of the depth gauge;
FIG. 2 is a perspective view of the depth gauge;
FIG. 3 is a front view of the depth gauge;
FIG. 4 is a back view of the depth gauge;
FIG. 5 is a side view of the depth gauge;
FIG. 6 is a side view of the depth gauge showing the side opposite that shown in FIG. 5;
FIG. 7 is a top view of the depth gauge; and,
FIG. 8 is a bottom view of the depth gauge.
The ornamental design which is claimed is shown in solid lines in the drawings. Broken lines form no part of the claimed design. The dashed lines shown in the drawings illustrate general environmental features.