If he didn't have extreme heart he'd have quit after being hospitalized by Penn after their first fight. Or after being choked out by Hughes. Or finished by Serra. Or knocked down by Condit. Or wouldn't have come back to win the last round against Hendricks. Talk to any coach of any contact sport, they'll tell you most people quit long before they even get to that point.
More to the point, no one without a lot of heart goes anywhere in professional sport. You lose to everyone when you first start playing a sport, you get battered around in practice and in games. You have to sacrifice constantly, work out when you don't feel like it, accept losses and move through them. Most people give up long before they get to that point.
Anderson tapped in his physical prime, as GSP did. Fedor tapped later on, but still tapped. If not going out on your shield is a sign of lack of heart, then Jones is the only one who qualifies. And Jones was caught using PED's, which is hardly a sign of having heart (ie in believing you can win without them). The reality is though that they all had far more heart than 99% of the population. Most people quit long before they make it to professional ranks in any sport, because they lack the heart to train that hard and on days when they'd rather be relaxing, to work through all the bad days of training when everything goes wrong.
There's a lot of criticisms you can make about GSP or any pro athlete, but not having extreme heart simply doesn't apply. People with average heart simply don't get far enough that you'd have heard of them.