Wow, your really have no clue what your talking about as the plasma gun is superior in every way to grav guns.
lets compare the two versus a Trygon:
A grav gun versus a Trygon. The Trygon has a 3+ armor save, so the Grav gun wounds on a 3+ and ignores the Trygon's armor, but the grav gun only has a 9 inch range if the wielder moves.
A plasma gun versus a Trygon. The Trygon has a toughness of 6 and the plasma gun is strength 7, so the plasma gun wounds on a 3+ and ignores the Trygon's armor, however the plasma gun has a 12 inch range if the wielder moves.
Winner versus the Trygon: The Plasma Gun wins as it preforms identically, wounding on 3+ and ignores it's armor but has a superior range.
Lets compare the two versus a squad of Guardsmen:
A grav gun versus a squad of Guardsmen. The Guardsman have a 5+ armor save so the Grav gun wounds on a 5+ and ignores the Gaurdsman's armor, but the grav gun only has a 9 inch range if the wielder moves.
A plasma gun versus a squad of Guardsmen. The squad of Guardsman have a toughness of 3 and the plasma gun is strength 7, so the plasma gun wounds on a 2+ and penetrates the Guardsman's armor, however the plasma gun has a 12 inch range if the wielder moves.
Winner versus the Guardsman: The Plasma Gun is superior in every way versus lighter infantry wounding them on 2+ versus the grav guns 5+ and has better range while mobile, so it clearly wins.
Lets compare the two versus a Rhino:
A grav gun versus a Rhino. Glances on a 6, but the grav gun only has a 9 inch range if the wielder moves.
A plasma gun versus a Rhino. It glances on a 4 and penetrates on a 5+ and the plasma gun has a 12 inch range if the wielder moves.
Winner versus the Rhino: Again the Plasma gun is the clear winner.
Lets compare the two versus Marines:
A grav gun versus a Marine. The Marine has a 3+ armor save so the Grav gun wounds on a 3+ and ignores the Marine's armor, but the grav gun only has a 9 inch range if the wielder moves.
A plasma gun versus a Marine. The Marine has a toughness of 4 and the plasma gun is strength 7, so the plasma gun wounds on a 2+ and ignores the Marine's armor, however the plasma gun has a 12 inch range if the wielder moves.
Winner versus the Marine: Again the Plasma gun wins.
So yeah, how is the Grav Gun better? Also, good luck making it to within 9 inches of the Riptide to use those grav rifles.