The gravel i use is small enough (i assume so) that if swallowed it will be easily digested by my axies.
Like mil said you could us a slat platform to feed them from and train them to take the food from there (but then u gotta worry about the current, if any, moving the food off it), or train them to take from your hand.
I got 2 adults yesterday and to feed them i just stuck in an old plate which some pellets on it. This of course was near a current so the pellets floated off, i then moved it further back and it was fine. All you gotta do is just lead em to the food or let them find it for themselves.
I feed my juvi's by hand so they rarely look in the ground now. Sometimes they will nibble at the pellets others they will gulp it in whole.
With my adults now i will be feeding them earth worms as a staple. Basically dangle it infront of the axies face and whamo, axie got a mouthful of worm.
Also like mik said different substrates to use. Sand is easily digested as sometimes pebbles may get lodged in the digestive tract of the axie. If you do use gravel make sure it is bigger then the axies mouth or small enough that you think it will be passed.