Run what you brung.
As they say, "if you're expecting trouble, bring a rifle". Why would you not want the most effective tool available in a situation like this.
Even easier to mount a MG on your vehicle.
A rifle is a good compromise, but only really optimal at medium range. Indoors you probably want a shorter weapon. In space you want a recoilless weapon.
Since a gauss rifle would be banned more or less anywhere with a measurable law level, you can't expect to be able to carry them outside of wilderness, or possibly the starport. A hunting rifle or shotgun is much more likely to be legal, if perhaps a bit conspicuous in startown...
An SMG is more convenient to carry, and a handgun much more convenient. A weapon you actually brought is much better than an inconvenient weapon you didn't bring.
If you really expect trouble you may want heavier weapons...
By this time, there are billions of GRs scattered hither and yon across the imperium. Unless the local infrastructure simply can't support them (magazines break, can't be recharged, ammo is only made on that ONE world that's the Imperial Armory), I'd expect to find them everywhere that has contact with Imperial travelers.
Agreed, there will be piles of surplus gauss rifles available, just as there will be piles of surplus laser rifles, ACRs, and assault rifles from lower tech worlds.
And Travellers will probably prefer gauss weapons since they are measurably (slightly) better. Unless they expect to use them in space.
But that also means that they and their enemies really will want to find some reasonable armour. Running around naked in a high-tech firefight is not advisable. I expect there will be reasonably armoured surplus high-tech vacc suits available from the same Imperial sources.
And when everyone has armour you haven't changed much except created a little bit of power inflation in the game.
But if the players insist on carrying gauss rifles everywhere they can, I wouldn't try to stop them, just make sure the opposition is also reasonably armed.