Unless I am mistaken, the one fundamental and visual difference between the Gato and Balao classes is the way that the periscope shears were built.
The Gato has a fairly substantial external support structure used to hold and support the two periscopes. This is a somewhat boxy looking set of "T" shaped support beams.
The Balao replaced all that external structure with a bascially self-supporting pair of conical housings for the two scopes.
I would consider this to be the one consistent visual differentiator.