The difference between gou and diao and is that gou focuses on the middle, ring and pinky fingers, while diao focuses on the thumb, index and middle fingers. Gou has more of a "sliding/leading" quality while diao has more of a pinching or grabbing quality. That being said, gou often immediately precedes diao. You can go for a gou and if you don't get it, circle around the opponent's arm into a diao, sliding down as far as the wrist, where you can pinch the joint between the wrist and hand.
Keep in mind that gou-lou-cai-gua and diao-jin-beng-da are in some ways like a parallel set and that in addition to their usual meanings of say, "beng" being a beng-chui, the characters can also have meanings in terms of strategy. Gou can help you set up a diao, lou is a means to jin, gou and lou give you cai--a method to "beng" or "break open" the enemy's defense, setting up for gua (taking the opponent down) or da (any strike).
What senses do we lack that we cannot see or hear another world all around us?
--The Orange Catholic Bible