Once you get past the conning tower configurations between Gato boats and Balao boats there are few, if any external differnces. Much of the difference dealt with the thickness of the steel in the pressure hull - and the resultant diving depth that resulted from the stronger hull. Perhaps a more important external difference (both Gato or Balao) was the limber hole pattern which varied between the building yards. There were radar fit differences - but some Gato boats received updates during refits
Revell of Gremany will be releasing a 1:72 WWII USN Fleet sub this November. It will be based on the USS Cobia at Manitowoc, WI. I don't have the information readily at hand, but I believe that the Cobia's conning tower configuration is what is called 'Covered Wagon'. Nautilus Models is ready to release aftermarket conning towers in Gato-as launched, cutdown, and post-war versions.
Revell's Lionfish (1:178 scale) is currently available. Nautilus Models makes replacement conning towers and laser engraved wooden decks in the above formats - as well as the Covered Wagon style.
Trumpeter has listed in their recent magazine ads a 1:144 scale as-launched Gato. They will also be doing a later version with (IIRC) a cut down CT. Expect the first kit to be available within the next month or two. The second's date is TBA.
Toms Modelworks used to offer a 1:350 scale resin & brass Gato/Balao fleet sub. It is very nice. Similarly, Yankee Modelworks offers resin & brass fleet sub. They come in several flavors - as launched to cut down. Both the Toms and Yankee subs are whole hull
Classic Warships made a waterline 1:350 scale fleet sub. It was one of the early offerings. It came with no brass and there was no limber hole pattern on the side. It is OOP.
PitRoad/Combat Sub also offers a 1:350 scale resin & brass fleet sub.
PitRoad/Skywave made a 1:700 scale plastic Gato. It used to come two in a pack with a Japanese subchaser. The kit also was included in a PitRoad Japanese DE kit. Tamiya has obtained the PitRoad kit and has released it under their label.
The PitRoad 1:700 scale sub has a rather clunky CT. Cut it back and replace it with some Evergreen - it looks really nice. Toms Modelworks has a PE set for the kit. Loose Cannon offers a suite of replacement conning towers for it.