I don't have an opinion about Gonzales for SCOTUS. For AG, however, he's got the right mix of skills and there's no evidence that he's a liberal or even a squishy moderate, except on the one issue of racial preferences. If I was him I'm not sure I'd take the job. AGs in second terms are usually called upon to investigate the White House, and can almost never resist. Having come from the White House it will be even harder for Gonzales to resist investigations if he wants to avoid being called an apologist or shill for the President.
He'll take the job for the same reason Bush offered it, they are friends.