Gary Oldman is so good as Commission Gordon that you don't really notice him at all. And, yes, that's a real compliment.
You can tell Oldman is outstanding, in short, because he doesn't stand out. It's a good trick, since of all the characters in the movie, his is perhaps the moral center.
Like Harvey Dent, he's a law and order kind of guy. Like Batman, he's not afraid of unorthodox approaches to a problem. But the closest character of comparison for Captain, er, I mean, Commissioner Gordon, is Assistant District Attorney Rachel Dawes.
Both Dawes and Gordon are civilian law enforcement officials who are faced with a choice between ordinary and extraordinary methods of justices, as represented by Harvey Dent and the Batman, respectively. For Rachel, a choice must be made; you can't have your cake and eat it, too, and her romantic relationships with Harvey and Bruce highlight this dilemma.
But, for all his being a good guy, Gordon isn't an idealistic; he's a pragmatist and an opportunist. He doesn't see Harvey Dent and Batman, and what they represent, as two things between which he must choose. He sees them as two useful tools for accomplishing his mission, each with its pros and cons. To Gordon, Harvey Dent (ordinary justice) and Batman (extraordinary justice) are just two sides of the same coin.
The script makes a point of positioning Gordon as more flexible, less extremely hard-nosed than Harvey Dent. It's kind of subtle, but several times he and Gordon are in conflict about the fact all the men is Gordon's hand-picked unit don't have exactly spotless records. That's unacceptable to Harvey Dent; after all, you're either all-good or all-bad, right? Small wonder the cops call the unforgiving Dent "Two-Face" behind his back.
Gordon's a bit more practical than that. The issue is underlined later when Detective Montoya, oops, I mean, Ramirez, confesses how she was suborned by the gangsters when in desperate need for Money For Her Sick Aunt May (or some such). Is Ramirez a bad guy? No, she's essentially a good guy trying to cope with bad circumstances (like many citizens of Gotham). Gordon understands that better than Harvey does.
Gordon's real test isn't choosing between Harvey Dent and Batman (which he doesn't do). It's in deciding to fake his own death and in cooperating with the conspiracy of silence about Two-Face's misdeeds. But even in those decisions it's just about having to hurt his family and his friend Batman in the process, not any intrinsic moral principles.
James Gordon, pragmatist; he bends his principles, so he doesn't worry about breaking them. That's what makes him successful in the films and makes him the character most immune to the Joker's various machinations in the film.
