Always the more the better when it comes to water fasting. You will be doing great things while drinking just water, but (and there's always a but :) )
Try not to over do the water because anyone who drinks too much water can get water toxic syndrome. A gallon sounds about right. I drink about a gallon if not a gallon and a half a day while fasting with water and I have never had anything wrong with me.
I am on my 10th day today and feel great!
Keep up the good work and remember that too little can cause dehydration and too much will become water toxic. :)