GURPS gives rules for implanted/mounted weapons. They aren't very good.
As described in the basic set, and elaborated on by Ultra-Tech, Extra Arm (weapon mount) gives exactly that; a thing into which you can strap a gun and use it without taking up a hand. Ultra-Tech talks about weapon mounts as being internal, but there is no bonus to concealment. Having a gun on a weapon mount is identical to having a gun in a holster in standard clothing.
Therefore, it's my thought that weapon mount does not, on it's own, give an implanted weapon. A bulk -5 rifle that is still at -5 to holdout is not hidden.
There are a few options; you can treat weapon mount guns listed as implanted as having the concealment clothing bonus of +4 to holdout. Or, payload. Each level of payload gives BL/10 of storage space. My thought is that you can mount the weapon internally by purchasing a payload, and then a weapon mount to put inside of that payload. Payload also does not confer any bonuses, but it seems that it would make sense to give the payload +4 to holdout of concealment garments, at a minimum.