Im a builder.
Sorry, can't help...your draftsman is right, the gutter will run on too much of a grade for it to work, but if you are happy with a variable pitched roof and not conventional Hip/Rafter and Ridge then it can be built easily (depends on type of roofing of course) with a gutter that runs correctly. What you can't see is an elevation of the boundary wall, it'll look stuffed with a conventionally pitched roof, on a big angle.
Why the 200mm offset to the boundary? I'd build it so the gutter sits against the boundary to maximise my site.
200mm to the wall then plus Facia and gutter, so am losing 50mm ish. By variable pitched are you talking about the rear hip being skewed as per my awesome model? Another crap thing is how if you look at the front of the garage I protrude 500mm into the neighbours block. When they subdivided my block they cut around the garage instead of having a straight line. Btw it's going to be just iron roof