In rock-climbing, we describe an incline as “steep” (which is anything from 90 degrees to overhanging) but we don’t have a good word to mean “not steep”. At least I’ve never heard anyone use one; instead people use convoluted phrases like “not too steep”.
I looked at the “opposite for ‘steep learning curve’” post, but none of those suggestions really seems to fit here.
Edit: I asked a rock-climbing English major and she said that “low-angle” and “slabby” were most often used for cliffs in the 50-90 degree range.
I’d say shallow or gentle are suitable…