The ornamental design for a gridstructure, as shown and described.
FIG. 1 is a top right side perspective view of a grid structure according
to the present invention, the opposite side being a mirror image of the
right side; and,
FIG. 2 is a bottom left side perspective view of the grid structure, the
opposite side being a mirror image of the left side.
The characteristic feature of the design of a grid structure according to
the present invention resides in the fact that the interpenetrating grid
sections, which form the interstices defining the grid structure, have
sinuous surfaces.