I agree the pay difference between low and high level fighters is important, but GSP started out being low paid too, but kept at it. In fact that's true for almost every fighter who becomes a champ (the exceptions being Brock and Rousey, who the UFC wanted from day one).
I don't think you can divide heart between taking punishment in training vs taking punishment in an actual fight the way you do. It feels the same in both cases. Its possible that GSP quite as soon as things got rough in sparring, but I very much doubt it -- he was too successful in the cage for that.
Moreover, he kept going through some pretty rough handling in the cage. He was hospitalized after the first Penn fight (GSP skipped the press conference to go straight to the hospital, something Hendricks didn't come close to doing to him). He was hit as least as hard by Condit'd kick as by Serra's punch, but held on (I suspect because his camp taught him what to do in that situation after the Serra fight). He came back against Hendricks in the last round (I think he still should have lost 48-47 but he definitely won the last round) when a quitter would have just packed it in (either just running away the whole round or even quitting on the stool) instead of digging deep to find a way to win the round. How about continuing the fight against Shields despite losing vision to eye-pokes? Why not just quit at that point, like 99% of us would have done? As tough as most Sherdog posters are, I don't see most of us wanting to fight Shields when we couldn't see much of what was happening.
GSP did quit twice, against Hughes and against Serra. I suspect both were because he didn't know what to do. There's probably an escape to Hughes' choke (I don't remember off hand) that he missed, but I suspect he didn't know it. Same for blacking out from Serra's punches, there are things he could have done (and later did to against Condit), but he probably didn't know. Its the same for Fedor and Anderson tapping out -- sometimes you don't know how to escape. Did they lack heart as well?