I assume you have set the roof elevation to 0 and still have the gap.
The easiest way is to have a separate level for the roof and adjust the elevation of the level so there's no gap. If you have an elevation > 0 for the roof you have to adjust that elevation.

For the roof to fit on a sloping wall you have to adjust both the height and the angle. Changing one will also change the other so it's a matter of experimenting with both numbers to get it right. If you're close you can also adjust the wall slope a little to make it fit perfecty.