I have the GTI-1500 and it is similar to the AT Pro, as is the GTI-2500. I think the main difference is that the AT-Pro is more waterproof. I cannot use mine in the rain without protection. The GTI's will work with the coil under water, but not the thinking end. Mine locates to a depth of six to twelve inches. However this always depends on the type of soil, the size of the object and the type of metal targeted. I found a horse shoe that was about 15 inches down. It was one of those large, heavy shoes like they toss in the game.
It is a more complicated machine, so be sure to practice learning how it discriminates. Practice on known targets, both good and bad.