OK... it's hard to follow without a program written to do it for me, but I believe I've traced the branches the Gauss ammo can take that end up in shops. I've ignored collections that have "loot" in the ID, since those appear to be designed for flashpoint rewards and the like.
It does not appear to be possible for them to show up in faction or black market shops. Any normal shop at a planet with the Star League tag has a chance to get them, though. If a standard Gauss Rifle shows up, there will always be a single ammo bin available as well, but the ammo can show up by itself. (The + and ++ versions don't come with ammo.) All of the following planets have the relevant tag and include the itemCollection_shop_starleague bit which appears to be the only path to Gauss ammo:
- Andarmax
- Bellatrix
- Brisbane
- Brixtana
- Cate's Hold
- Cavalor
- Claybrooke
- Dunianshire
- Hurik
- Illiushin
- Jamestown
- Joppa
- Kimi
- Midale
- New Roland
- New Vandenburg
- Niomede
- Piriapolis
- Repulse
- Smithon
- Victoria
- Yuris
- Zathras
I haven't made any attempt yet to figure out what the odds end up being, but it seems likely that the systems with fewer OTHER item collections available are more likely... and 7 systems have only a single other collection:
- Anadarmax has itemCollection_major_Liao
- Brixtana has itemCollection_minor_MagistracyOfCanopus
- Kimi has itemCollection_minor_Locals
- Piriapolis has itemCollection_minor_Marik
- Repulse has itemCollection_minor_Liao (probably fewer other items possible than at Anadarmax)
- Smithon changes depending on who has control... when the Directorate is in control, the only other collection is itemCollection_minor_AuriganDirectorate
- (Smithon under the Restoration has itemCollection_minor_AuriganRestoration and itemCollection_MechParts_RestoSmithon)
- Zathras has itemCollection_minor_Locals