What is the opposite of "steep"?
Solution 1:
I'd say shallow or gentle are suitable...
Solution 2:
Perhaps one of these?
- Flat
- Gentle
If a noun works for your usage, you could go with glacis which means "gentle slope"
Solution 3:
I would suggest you can call it "a gradual incline."
Solution 4:
In the context of inclines, I suggest that 'slight' is most commonly used as the opposite of steep.