I have used gravel in the bottom layer of a viv in the past. Using hydroton is much easier to work with in my opinion. Your tank comes out much lighter than when gravel is used, plus you can use less. Gravel will work though. I put the sphagnum moss below the eco earth and leaf litter on top though. That just lowers the risk of your frog getting a mouthful of it but helps maintain the humidity. I would also recommend putting some screen down so that the eco earth doesn't just end up mixing with whatever you use for drainage. Other people here use egg crate and skip any kind of gravel or hydroton altogether, that seems to work well. Make sure you either put a pump in the bottom layer or some tubing for siphoning to drain excess water if you can't drill a hole to do that.