FIG. 1 is a perspective view of a gas cooktop showing my ornamental design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof; and,
FIG. 7 is a bottom view thereof.