We have grav weapons in one of JTASes, JTAS 4 if I am not mistaken, although I dont like the writeup very much as author seems to confuse mass and weight at times. It is listed as TL 13-14.
Anyway, the core of my issue is that as written, weapon design rules should allow applying "Rocket" modifier to "Gauss" rounds. An idea was flung above by you, Kranth-Technoshaman, that gauss gives the initial boost of speed, but that would negate "Zero-G", so I was trying to come up with fluff to support this mechanical construct - a gauss, but rocket - that would not be self contradictory - hence the assumption that the rocket uses gauss-like thingie for propulsion - i.e. less fuel, so rounds are smaller, so you have more of them in the mag. The receiver is larger, because it needs to keep the rounds charged at all times. So this kind of fluff explains all the mechanical effects we get for applying Rocket to Gauss as base. Just needs a good name.
To all other responders - see above - the discussion is about explaining a mechanical thing in the rules offered, that is probably unintended, namely applying "Rocket" modifier to "Gauss" weapon base, as if it was a conventional round - i.e. double the price per round, reduce ammo to 60%, halve the base receiver mass, set range to 250 m, add Inaccurate (-1) and Zero-G. Pages 38-40 of Field Cat. In a way it produces a higher TL recoiless weapon. Just needs a good fluff and a name, if we are to use it.