I finally got my 1st Gato, skipping over Tambor/Gar (albeit a bit late: Nov 1942).
I guess it's like what @propbeanie said. You have to get good mission rating, not just total renown. Be aggressive. I sneaked into Truk again and this time, got shot at by a submarine. I saw the enemy torpedoes streaking by me and lucky I was deep enough to avoid it. I was running out of torps and heading home, but ran into two convoys with big escorts. I still attacked them with my remaining few weapons including one very long range shot on a fast moving merchant at 7000yrd (I am playing at 90% realism, only unchecked external view). Btw, a couple of more of observations:
1. last night I pulled into my base port at Brisbane and found many sinking ships and fires. Did it get attacked by Japanese? Most sinking ships have no visible damage (no holes on them), only one has a small fire on it. Brisbane is pretty far from the front, Japanese navy would venture that far? I hope it's not one of those anomalies caused by fragile installations and some file issues.
2. Sub skins: All American subs are painted in black. It's kinda dull since many classes look very similar. Would any later classes have different color paint?
3. How do you tell if some areas are protected by mines?
4. What do metals do now? I remember it used to increase experience, but I don't see anything happening.