Floating point coordinates. If you want to keep the gaps from showing up, add an event:
scroll to x: round(scrollx)
scroll to y: round(scrolly)
Either that or make them all overlap slightly (a pixel or two) if you want to keep the smooth scrolling.
Edit: It might not work if you're using the 'center screen' attribute. In that case you'll have to use:
Sprite: set x to round(sprite.x)
Sprite: set y to round(sprite.y)
Edit 2: whoops, thought you said sprites - right, overlapping the tiled backgrounds won't work, but the rounding still will.