What else did you do when you planted it? Did you soak the root ball for an hour or more after taking it out of the pot? And did you untangle the roots a bit and spread them out to avoid circling and girdling roots?

Did you plant the tree in just the native dirt, and position it fairly high without adding fertilizer or compost or other kinds of soil to the hole? How is the drainage?

I couldn't say if X-number of gallons of water are enough or too much. This isn't a particularly good time of year to plant trees, but sometimes needs must, and (full disclosure) I planted a whole bunch in my yard in July within months of moving into this house in 2002. I lost a couple of them then, probably because of the mistakes mentioned above - not soaking and unwrapping, and also probably by having them too deep and having not made the hole wide and shallow and spreading out the roots.

Trees sometimes look a bit stressed after they are planted. The amount of water you put on the surrounding turf should be enough for the tree, once it is established, if you water deeply a couple of times a week. During the next few hot months you may want to pour on water up to four times a week, supplementing rain and the lawn watering schedule.

If you didn't loosen the root ball go ahead and do it now - pull the tree back out and add some compost tea (Garrett Juice) to a bucket of water and let it soak and loosen that dirt from around the roots in the pot. Sometimes the dirt is so compacted that it acts like an impermeable pot itself, not letting water soak into the roots no matter how much you water. That's why you need to untangle the roots, be sure you've removed all of the extra dirt from the flare, and reposition it in the hole before covering with the native dirt.

Good luck!

Overwatering is as big a problem (perhaps bigger) than overwatering.