I would be looking very closely at the soundness of such advice.
One gallon of water plus the significant water content of 13 fruits and vegetables is a lot of water, and might lead to losing too many nutrients. Contrary to previous advice, one ought to count the water content in all foods and liquids as part of the daily need.

Second, concentrating exclusively on a single category might put you way behind in other categories, especially protein, which is very important to maintaining muscle mass.

Here are the most current dietary guidelines, expressed in ideal servings per week, and depending on age: https://www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf
Most extreme diets are unsustainable, and it's hard to get the variety of nutrients required across the board.
Some of the most important guidance I ever received, 50 years ago, was the USDA food pyramid, concisely showed the "nutrient density" of a variety of food- vitamins, minerals, protein and fiber compared to the calories. Another was my Grandma's philosophy- the more colors on the plate, the better for you. She was a farm kid without formal education, but she and my grandad and all their kids lived well past 80 in spite of many serious health conditions following this plan.

Have you been offered any evidence that this is a researched and accepted strategy for avoiding cancer?
Sue