Helo, wanna ask how important isit to have a gradient for drainage pipe?

contractor told me that its not that easy to clog , as the pipe being 42mm dismeter and short length ( total about 11m with 2-3 bends ( like a U shape ) to manhole.. and also limitation of few parts of the ground, floor too keras to hack through ( ground floor )

But based on what i read online drainage suppose to have gradient running at 1/80 or 1/110