Do you Sweat at the Gym?
Even if you do not, you are most likely training in the comforts of air-conditioning. Air condition causes a constant loss of water through the skin as the atmosphere is generally dry, and will find water where it can get it. Your skin will willingly give it as water is being drawn to the lack of heat at the surface.
So How Much Water Should One Drink in this Instance Then?
I would personally recommend a minimum of ten glasses of water on top of what you are drinking and eating. In short, this is around one serving of water per hour in the form of food, supplements or plain water.