Welcome to BYC. Where, in general, are you? Climate matters.
Gravel is almost always a terrible idea for a chicken coop.
It's hard on the chickens' feet -- making it more likely for them to get bumblefoot -- and the poop filters down into it and stays until it gets wet, rots, and REEKS.
If you're having a mud problem it would be better to get coarse wood chips, the sort you get from a tree-trimming service. Alternately, coarse, undyed mulch.