The ornamental design for a gripattachment, as shown and described.
FIG. 1 is a top, front, right perspective view of a grip attachment showing our new design;
FIG. 2 is 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 broken lines immediately adjacent to the shaded areas define the bounds of the claimed design and form no part thereof. The broken lines depicting the remainder of the grip attachment show features that form no part of the claimed design.