All in my not so humble opinion of course...
You could still use a gauss weapon. Just make a folding light gauss rifle with a subsize round and a separate battery system to the clip. Gauss needles are already really small, do the above and you might go down as far as 2g/shot. Means a 500 round clip weighs a stupendously heavy single kilo.
The M6 uses .22 and 410 shells, neither of which are terribly powerful, a short light gauss weapon would be of a kind.
How about a rocket round instead. Short barrel, rocket assisted heavy calibre round. Could use variety of loads. Just ideal for dealing with T-Rex analogues.
I guess that's the point
What is the definition of a "Survival" weapon?
Do you need the long range, high armor penetration and high autofire and round capacity that a gauss rifle gives you?
Do you need the stopping power of a bazooka?
Or is a simple varmint rifle and light bird-shotgun enough?
Depends on what's for dinner on the planet you find yourself marooned on
But let's not try to make this some uber weapon. It shouldn't be your first choice when Imperial Marines in Battle Dress come knocking at the hatch for a look around. And you shouldn't be thinking it's the one you want to take on that sport hunting trip to Jurassic Safari World.
It should be the simple, small, light, cheap weapon you wouldn't want to use for big game hunting but don't mind always having stowed under the seat in the air/raft or in the ship's locker as part of the survival kit. It should be something that is hard to break with few moving parts and none of them delicate. It should be loosely enough engineered that you can pour sand through it, drop it in the mud, and still shoot it.
The M6 fits the bill to a T in my opinion. Well, I'm not sure about the sand and mud tolerances bit but it should be.
You should also be thinking it's not something to keep you alive from all harm for years and years. Able to put down a dangerous large predator or hold off a small army. You should be thinking just enough to get you some small game to eat and maybe make some noise to signal the rescuers who you hope are coming before too long.
Trent's original idea of using found items for ammo isn't bad. Though I'd rather not have to be looking for ammo and wondering how well it'll work. And I'm just not sure it marries well with gauss/rail tech. It all sounds expensive and fussy to me. But as I said both of those are just problems (one's marketing, one's engineering) to work out if you want the idea in your game.