The ornamental design for a gameboard, as shown and described.
FIG. 1 is a front perspective view of a game board showing my new design;
FIG. 2 is a front side view thereof;
FIG. 3 is a rear side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top side view thereof;
FIG. 7 is a bottom side view thereof; and,
FIG. 8 is a perspective view thereof while the game board is being used.
The dash-dot-dot-dash lines in the drawings define the bounds of the claimed design while all other broken lines in the drawings are directed to environment and form no part of the claimed design.