So if my Greninja uses grass knot and Gravity is up(down?), will grass knot become more powerful since the weight is technically increased?
Gravity does not affect weight of the Pokemon. Its gets rid of any Ground type Immunity such as Flying types immunity, Levitate, or airbloon. It also will bring any airborne Pokemon down when it uses a move like Fly or Bounce and prevents the use of them at all. It also blocks Hi Jump Kick from being used.