The ornamental design for a gateway, as shown and described.
FIG. 1 is a front perspective view of a gateway showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The unshaded areas, including those unshaded areas within the triangular shapes on the top and bottom of the device, form no part of the claimed design. The broken lines depicting the remainder of the gateway form no part of the claimed design.