The gray Hulk, the baser instincts of Bruce Banner, was the first version of the Hulk that the public witnessed. Perhaps because of the time relativity- the gray Hulk is the physical representation of Bruce Banner's adolescence. That is why the gray Hulk posesses a selfish sarcastic attitude- a somewhat typical teenager from hell attitude.
The gray Hulk's morales are somewhat more flexible than most of his other counterparts. In his first appearance the gray Hulk had decided to kill Rick Jones because he was the only one who knew his secret identity. He was only stopped by the sunrise. Gray Hulk uses his crafty mean streak to compensate for the fact that he isn't quite as strong as the savage green Hulk. The Abomination found this out, when after he compared gray Hulk to the savage green Hulk, Hulk doused him in toxic waste nearly killing his emrald foe. In fact one sure way to increase the gray Hulk's strength is to note that he is less impressive than his green counterpart. After all though not as strong he is still the Hulk and "the madder Hulk gets the stronger Hulk gets".
The gray Hulk also enjoyed his happiest times as a Las Vegas legbreaker Mr. Fixit. The sorcerors of Kai placed a spell on Hulk that temporarily allowed Hulk to remain during the daylight. He wore fine clothes, ate fine food, garnered respect and fear from all of Las Vegas. Hulk also engaged his more carnal desires as the most eligible bachelor in town. It was ironic that the emergence of Bruce Banner ruined things for the Hulk instead of the other way around.