How do I fit a drain gully when the drain pipe is very close to the surface?
Given that the water will only drain as low as the bottom of the drain pipe, you may have to put in a new drain pipe, or move the existing pipe so it's deeper.  If you're lucky, it has a steep runoff angle so you may only need to replace/move a few feet of it to get it to a better depth.

Beyond that, you can buy the gully hopper and build your own u trap, allowing the exit to be placed as high or low as needed.  Ideally it'll be several inches below the top of the gully, but it may be sufficient to have only an inch or two.

However, drainage away from the house is extraordinarily important, and if you're going to the trouble of lowering the paving, you should consider adjusting the drain pipe so the slopes and depths are correct for the new position of the gully.