...............any roofers out there..........

Im not a 'roofer', but have done some. I tinned my shed roof (very low pitch). I put a ridge vent on it. The gap under the vent sounds like a similar situation to what you are describing. I used foam rubber strips that are pre-cut to the contour of the tin. I think most places that sell the metal have the matched fill strips. I caulked them in place,and completely covered the exposed side with caulk. If possible, I would flash behind the gutter, and extend down over the tin - especially if it is ever to be a 'finished' room. As long as it all fits tight and gutter is solid, you can probably get a good seal under it.
Keith