Eh, the issue is Gauss Barrel and even the device on the side of the Machinist aren't unique per weapon because they are technology given to you during the job quest.

Pretty much the logic is, Gauss Barrel is one size fits all as is the device, this is different from quivers because a quiver isn't a unique piece of technology that was specifically designed for Machinist.

A Gauss Barrel is given to you, so it's not a magic piece of tech that molds into a new form based on the weapon you were using, that could suggest that each weapon would be designed with a unique Gauss Barrel attachment but in all honesty that would be illogical/waste of funds to not create tech that was one size fits all.

So yea it kinda sucks but it is lore wise however finishing the MCH questline I think it is reasonable to say that they may find a way to add in a quest that allows you to customize the look of your Gauss Barrel much like they plan to introduce glamour for summoner pets. Mean it could be a goofy quest where the Leader of the MCH guild is thinking of ways to encourage people to join and he comes up with an idea to allow people to customize the look of the Gauss Barrel cause some people might not like the fashion clashing with the gun they are using, so you are sent on a quest to get mats to test the efficiency and ease of creating a new Gauss Barrel that can be easily customized without losing the efficiency of it's ability lol.