So, it's a bit of a hack, but as long as you're not using dividers for anything else, I think this should do what you want (added to your function ListView:initGridView()):
self.gridview:setHorizontalDividerHeight(10)
self.gridview:addHorizontalDividerAbove(1, 1)
self.gridview:addHorizontalDividerAbove(1, #self.items+1)
function self.gridview:drawHorizontalDivider(x, y, width, height)
end

It would be nice to do this in a more straightforward way though, I'll give it some more thought!