Well, if the FGMP has an integral grav module then you could put a grav module on a VRFGG too if you wanted to I suppose.
One argument for RP fusion guns and the like is that you don't have to worry about carrying and running out of ammo, something that happens really fast with a VRFGG.
Yes, and no...the p/FGMP's use a gravitic generator not only because of
weight, but as cited by Mercenary: the primary reason is for
recoil compensation. It's still a man-portable weapon even if you shut off the generator - it only weighs 23 pounds for the heaviest, but to fire it you need to deal with a tremendous recoil.
But a VRF gun - well, that thing might weigh as much or little as an autocannon or a fusion gun, but the thing doesn't sound handy to wield at all.
It sounds just like it's described as: a heavy, pintel mounted support weapon as chunky as an autocannon. So logically you'd need something larger than a man to use it as a personal weapon. Unless, I suppose you are in something like W40K and everything is over the top.
There are workarounds - I have a G-SAW in the "Pimped some guns" thread that fills the gap handily without making unrealistic compromises. Of course if you want to make an argument for mounting these VRF's on a small "mech"-type suit then that might work. Just not IMTU because BD is cheaper and more versatile when it shows up at TL-13, and before that (and to a lot of extent, after) drones and tanks are just cheaper, fatser, and better protected.
On the other-other hand you might want a VRFGG for certain kinds of jobs. Fusion guns start fires and fires destroy things that you might want.
No doubt, just because it enters the production line before high energy weapons it doesn't mean you can't still use it. In fact the VRF makes a dandy coaxial or cupola mounted weapon for keeping the pesky anti-tank teams away from your grav tank when they are to close to use the main fusion z gun without scorching the paint.
Ok, marine locker yeah, but not your typical civilian merchant ship's locker . . . I hope?
Why would it be otherwise?
That is just because they didn't include weapons like the 12mm "Terra's Hammer" and the Vilani TL-15 10mm Gauss Rifle in the Striker weapons sheet! ; - )
Or powerguns ala' the Slammers which I added into MTU before Mercenary came out with P/FGMP guns, and still have them since they fill a gap at even TL-15. That's the beauty of Traveller - you can do what you want with it. If you want mechs, have mechs. You want different types of weapons than are in the admittedly sparse (but the basics are all there) weapons list for the OTU, then make them. I haven't seen a game yet that didn't do that. Check my early gauss weapons thread for some of my railgun designs for yet another example.