They were. We used basically the same sub throughout WWII, with improvements to hull strength and weaponry. There were actually three classes commonly used in the war, the GATO (1940), BALAO (3-42) and TENCH (2-44).
The pre-war TAMBOR, SARGO, SALMON, and PORPOISE classes were also commonly seen in the war, but not in anything like the same numbers as the GATOS and later boats.
USS Drum was the first of the GATOs to go to war, her first patrol beginning in April 1942. From Wikipedia:
Drum arrived at Pearl Harbor from the East Coast on 1 April 1942, and after a voyage to Midway Atoll, cleared Pearl Harbor on 14 April on her first war patrol. Cruising off the coast of Japan, she sank Mizuho on 2 May and afterwards endured a 16 hour depth charge attack consisting of 31 depth charges. Later that month she sank three cargo ships before returning to Pearl Harbor on 12 June to refit. Drum's second war patrol, which she made in the waters between Truk and Kavieng from 10 July-2 September, found her efforts frustrated by poor torpedo performance, but she damaged one freighter before returning to Midway to refit.
That should take care of any questions about the class being used "early war". Unless some clown decides that April 1942 isn't early enough.
I have t ostand corrected on the commission dates. Note that I cited '43 as the first commission date - that is what is listed here:
http://en.wikipedia.org/wiki/Gato_class_submarineHowever, shubert is entirely correct about the drum being commissioned before '43 - and Drum was the 12th Gato so there's no arguing that it's like the first four of the Gato class which were actually originally intended to be Balao class boats (go figure - since the later Balao class came after, not before).
The source on the commission dates is cited in footnote 2 of the wiki entry. I'm not sure what that author was smoking or if he qualified the comment. However, the boat was extremely effective as the war went on, probably in large part because of the improvement in american torpedoes.
In any case, I don't think anyone will argue that the Type VII is as good as the Gato .
Consider, first, a little comparo - The type VII is only about two-thirds the length and half the tonnage (based on the most numerous Type VIIC) of the Gato. It also has only half the torpedo tubes. The Gato was faster - on the surface, it's 21 knots compares pretty favorably to the Type VII's speed of 17.7kts - submerged, 9 compared to 7, and the Type VII was rated for less depth. Range was also less, but still long enough to be awful, unless you didn't mind the filthy, cramped quarters of the Type VII. Also note, the Gato was claimed to have some 48 hours of underwater endurance. The U-Booters were gasping at half that.
Second, the 77 Gatos sank 1.7M tons of Japanese shipping. About a fourth (21) of the class were lost during the war. The Type Vii, of which closer to 700 were built, accounted for some share of the 14M tons of allied shipping sunk but was considered obsolete by end of war - of the 1000 u-boats, all types built, something like 800 were lost. You do the ratios.
Of course, I realize you can't consider the weapon in isolation. The Type VII had arguably better opposition. I just think the VII a bit small and underarmed to be considered a late-war viable u-boat. The Type XXI, otoh... is another story.
