Drinking to your thirst' demand will be sufficient.
I think people who take in a gallon a day are doing so to circumvent their hunger and as a result of societies preconceived notion to drink copious amounts of water. '8 glasses a day!' But following that mantra, you best get in 3 heavy meals a day too.
Taking in excess water beyond what the body needs just gives the body more to deal with and will interfere with the fasting process. A quick google search will yield many results on 'death from drinking too much water' (don't worry, I don't think this is likely, but to illustrate my point in not needing to take in more than you need). Let your own body guide you in how much water you drink. Even though I find it unlikely, that could be as much as a gallon a day. And while water only fasting, your need for water will be considerably less than it would be normally.