GS = 50 + 3 · IP -2 · (H+R+ER) - BB + SO + (2 per each full inning completed starting with the 5th)
The game score is constructed in such a way that it can be calculated from a standard line score. It ignores balks and HBP and weights strikeouts twice as much as other outs. Nevertheless, it gives a solid measure of a pitcher's start and is handily calculated.
A GS above 50 may be considered a quality start (or above 60, if you're a bit more pretentious), 70+ is a genuine quality start, around 90 is really, really great and everything approaching 100 is immortal. Kerry Wood's 20K gem in 1998 had a game score of 105, the highest ever for a 9 inning game. Pedro Martinez 17K one-hitter at Yankee Stadium in 1999 had a GS of 98.