This is exactly the wrong approach that people take to this. It's not really an issue about availability so much as an issue about why folks are toey about them in the first place. Gauss rifles may or may not be legal to own in any given world but as of the Published 1105 setting they've been in production for something like a milennium, if not back into pre-Cleon days of the Sylean Federation. They are going to be on the grey market in quantity. There are even legitimate arms dealership companies like Interstellarms that get a mention in the Traveller source material.
They could be illegal locally, perhaps requiring some work to obtain, but the Imperium itself has no policy on trafficking anything smaller than WMDs and has a policy of turning a blind eye to local brushfire wars. Just within the grey market within Spinward marches one would expect to see many millions of Imperial or extra-Imperial manufacture (Zhodani, Sword World, Darrian, Aslan, Vargr, independent extra-Imperial polities) from several frontier wars and hundreds or thousands of local conflicts over the past centuries.
Under the RAW it's mass-murder on a stick. How is this possibly not the weapon of choice for low-rent gangland drive-by shootings or the shenanigans of whatever makes up the local equivalent of the mob or cartels?
That's a rhetorical question - the answer is 'of course it is'. I don't have an issue with Gauss Rifles per se, but the CT and Striker RAW have them crazy overpowered. The correct solution is not to make up contrived explanations about how parties at your table can't get hold of them because reasons. If you're going to do that they may as well not exist in the first place (which is also a legitimate approach, btw). The correct solution is to nerf them so they don't overbalance party-level fire combat scenarios. This is really a minor piece of house ruling to fix on a game that's famous for house ruling, and yet for some reason people seem unwilling to do it. I don't really understand why - maybe it's a canon thing.