I can't find a "real" way.. but one way to fake it is to use the Filters->Distort->Mosaic and pick the hex. You have to do this on a color layer.
Spacing = 1 (or whatever you want the line thickness to be)
Neatness = 1
Light = 160
Color = 0
Run and then use the select by color tool to select the inside color of the hexes and delete to alpha. Invert selection and fill with black (or whatever color you want the grid)