Is drinking a gallon and half of water a day bad for you?

Water. You should drink adequate water that is needed to quench your thirst. Drinking more than what you needed can certainly put your health in jeopardy including low sodium which potentially could cause seizure. However if you drinking that much water to quench your thirst on a daily basis, then you shoud get an evaluation, it can be a form of diabetes (mellitus or insipidus).
It depends. 64 ounces spread among 8 glasses is a classically recommended amount of daily water. Thus, 1.5 gallons is nearly 200 ounces and well above that amount. The primary risk of excessive water consumption is low sodium levels in your blood. If you are sweating a lot due to exercise and/or heat, then 200 oz. Could be appropriate. We ask some athletes to weigh before/after events to monitor fluid levels.