Actually, a better measure is based on your weight...recommended is 1 ounce for every 2 pounds of body weight. So a 200 pound person should drink at least 100 ounces/day.
I don't think a gallon is going to make you sick. The people that get "water poisoning" drink excessive amounts of water. I drank almost 2 gallons/day when I was pregnant and had no adverse reactions (other than using the bathroom every 30 minutes

).
The only thing I would caution if you drink lots of water (and/or live in a hot climate)...watch your electrolytes. You can get electrolyte powder to add to your water. It's tasteless and has no sugar, so it doesn't add calories. (Check out Electromix by Emergen-C.) Also, if you start getting muscle cramps, you may be low on potassium.