Does drinking a gallon of water a day flush out a uti?

No: Urine infection generally needs antibiotics. Bladder infection or cystitis may reslove spontaneously.Drinking water may alleviate some discomfort, not necessary to drink excess water.

Lowers germ concent.: Drinking lots of water dilutes bacterial concentration in urine. Causes u to urinate > often. Which helps bladder defense mechanisms to get rid of few remaining bacteria in bladder after voiding. No way as reliable as taking an antibiotic.