The bottom line: As long as Terra/Sol maintains the standards set with "Gauss Weapons", I'll buy any other entries in the "Six Guns" series without hesitation.
"Gauss Weapons" starts off with a few pages delving into the whats and hows of electromagnetically accelerated personal slug throwing weapons, including more detailed descriptions of the standard Traveller rules for these weapons (including a whole page on the batteries!) and additional rules and equipment options to make things more interesting. (This includes accessories and different types of ammunition that would work in any Traveller setting.)
The remaining 2/3 of the book is like a magazine review of a series of gauss weapons, from pistols and carbines up to assault rifles. These reviews include pictures, expert testimonials and opinions, and statistics (both "real world" sounding and actual rules). This section is quite amusing in many places, with comments like "such-and-such manufacturer markets this as a 'prestige weapon'...". Though the "fluff" references "Twilight Sector" specific corporations, there is nothing setting specific about the weapons themselves, and they would trivially drop into any other Traveller setting.
The final page of content contains a quick reference sheet of all of the relevant Mongoose Traveller statistics for the included weapons.