Just checked, wonderswan and game gear games are exactly the same width, so side clips in a GG case will definitely hold Wonderswan carts.
for retail dimensions, game gear is 15.5x11x2.5cd
wonderswan is 11x8.8x2cm
So, using the GG case for wonderswan is a bit overkill in size, but I would guess most peoples GG collections are larger than WS collections - certainly for me, I would have no concerns using a retail sized GG case for my WS. If the GG cases end up genesis sized, I would probably leave them in DS cases.