"Grand theft" is a legal term defined, for example in California's penal code here, as:

Grand theft is theft committed in any of the following cases

(a) When the money, labor, or real or personal property taken is of a value exceeding nine hundred fifty dollars ($950), except as provided in subdivision (b).

(b) [special cases omitted here]

(c) When the property is taken from the person of another.

(d) When the property taken is any of the following: (1) An automobile (2) A firearm.

"Grand theft auto" seems to be a colloquial abbreviation of of case (d)(1) in the list above: "grand theft of an automobile". It seems have started off as law enforcement jargon, later popularized by the 1977 film of the same name (and later, of course, by the video game).

It doesn't seem to be the official legal name of a specific offense, at least in California, but rather a piece of jargon widely adopted by law enforcement and the media.