Is a gallon of water too much to drink?

YES

but it is very difficult, you need to drink more then 10 liters per hour for several hours. you will probably throw up way before it becomes a problem. drinking too much water often occers when you drink alcohol as well as take extacy, the combination scrambles up your body heat control and body temp' can rise to leathel levels. in an atempt to reduce the temp one might drink too much water which can dilute the blood, damage the stomach and in extreme cases kill you.