White balance is how the cameras imager sees the color white...................as the lighting in your subject area changes so does the color balance and the camera either needs to see the color white manually (via White Balance card) or it will adjust to a preset factory setting that may or may not reflect the true color of the environment.I always,always set the white balance manually and continue to re-adjust it as the lighting changes.

Gain is a setting inmost pro-sumer camera's (and some consumer models) that can be adjusted manually to let in more light in low light conditions.......the more gain the grainier the footage gets.........to me anything over 6db is going to be unuseable footage, but some say up to 12db. Matter of fact I passed on a 128" 8 point this year because I had the iris wide open and the gain up to 12db. and the footage was too grainy in my opinion, so I did not shoot the buck, even though it was a slam dunk 15 yard shot.................