Can drinking 8 glasses of water a day make you gain weight?

Not long term. If you drink water and climb on a scale you will have more "water" weight, which disappears over time.Since water has no caloric value it will not add any weight in the long term.
Wt but not fat. Water is heavy and if you drink 16 ounces of water you will gain 16 ounces of weight = 1 pound. However you will simply excrete any extra water and your wieght will normalize. It is body fat that you should be concerned with.