That would make for some very large gutters. Is this project in Seattle?(lots of rain) Have you thought of what might happen if snow and ice builds up in these large gutters(lots of weight)? Just a thought :)
Anyway, it is possible to do as you are asking. Keep plenty of tips on hand for the plasma torch, as that is a lot of cutting(4500 lin ft of cutting = 2250 feet times two cuts). Layout will take a minute or two to make sure each half is equal to the other. A framing square and a bubble protractor will help in finding the centerline of each pipe to divide equally. Someone else may suggest another method, but this comes to mind at the moment. Plasma over Oxy/Fuel I think is a wise choice since you need to limit your heat input to keep the pipe straight and by making cuts on two sides exactly opposite each other will further enhance your chances of keeping it straight. "Buggo" makes a magnetic track that the torch can be mounted on and long straight cuts are easily achieved with very little grinding or cleaning up.
As far as a fabricator being able to handle it... I would think as long as the fabricator has time allowed for the layout and cutting, with some possibilty of some straightening, I would think most could handle it. I work for a structural fabricator and we will cater to a customer provided the cash is there to cover it. :)
Good Luck with your pipe-gutter project, and please post your results when you have completed it and let us here on the forum know how it turned out.
John Wright