What is claimed is the ornamental design for a boat gauge cover, as shown and described.
FIG. 1 illustrates a front view of a boat gauge cover incorporating my new design;
FIG. 2 illustrates a rear view of a boat gauge cover incorporating my new design;
FIG. 3 illustrates a left side view of a boat gauge cover incorporating my new design;
FIG. 4 illustrates a right side view of a boat gauge cover incorporating my new design
FIG. 5 illustrates a top view of a boat gauge cover incorporating my new design;
FIG. 6 illustrates a bottom view of a boat gauge cover incorporating my new design;
FIG. 7 is a front perspective view of a boat gauge cover incorporating my new design;
FIG. 8 is a rear perspective view of a boat gauge cover incorporating my new design; and,
FIG. 9 is front perspective view of three boat gauge covers of the present design installed on a boat.
The broken lines in the drawings that represent the portions of a boat are for the purpose of illustrating environment that forms no part of the claimed design.