American gags are generally used on very strong horses as they are fairly severe. Have more leverage than a dutch gag as longer shanks so tend to lift head up and in. Should really be ridden with two reins, could use roundings instead but this will lessen the effect.
There are a few good bit websites if you look on google, can't remember any names, maybe try Heather's bit bank i think that gives info on different bit types.
i tried this on my tb once for xc-its meant to be very severe but it didnt seem to make much difference!(i rode with two reins)Mine cut his mouth on it, it had a lozenge in the middle and i don think he liked it but i know some horses who go well in it. Hope this helps!