Our Scooter is quite a good talker. In fact, he was not supposed to be named "Scooter", it was a nickname we were using while we dithered over "real" names. Then one day he said "Pretty Scoo-bird" and he was named.
Some of his vocabulary other people wouldn't understand, but some of it is VERY clear. He does talk in context as well, fairly frequently. He's taken to saying "You're a sleepy bird!" when I'm ignoring him. This morning he said, for the first time that I can recall, "Come here, Scoo" when he wanted me to pick him up. Occasionally he says "Thank you" when I give him something to eat. He's very clear with "Pretty Scoo-bird" and "Hey Scoo!" among other simple phrases and he wolf whistes and does a 'come here" whistle. In his own, somewhat harder to understand way, he's as good a talker as our Cape is. He impresses the heck out of me regularly. We had not thought that he would talk when we got him, but he almost always talks "people" when he wants our attention. I keep wanting to get a sound-activated recorder in the bird room because he will rarely vocalize with a camera pointing at him.
I think one thing that encourages talking is to talk to them a lot. We never intended to teach speech, but the phrases he picked up first were ones we used with enthusiasm around him.
So, yes, GCC certainly CAN talk. I don't know how common it is for a GCC to talk as much as Scooter does, though. They are amusing birds, but high energy and rather prone to being bossy. People tend to think they are easy because they are small, but Scooter is more complicated in many ways than our larger bird.