The manual advises that the gap be filled with body filler and then sanded back 2mm before coating with gel.
Having done this it is just a case of sanding it smooth and flat with wet and dry and polishing up. The problem I had here is that I always seemed to sand back down to the filler by the time I was happy with the finish.
So my recommendation is just to build it up in gel coat and not bother with the filler. Having done this things became a lot more straight forward.
Then back to the wet and dry again
Followed by the polish