When has the gather occurred?

[Coach Bill]

I'd like to know when rule-wise the gather of a dribble has occurred? One of my guys does a drop-step where he takes one or two dribbles to the middle then simultaneous with picking the ball up, drops his front foot then his back and goes up strong. Is this a travel? If you rule he has gathered while his back foot is down, then it's a travel, of course. But, i don't know according to the rules when a gather has occured. I look at it similar to a Euro. Thanks!