The title shows up throughout the novel in various forms—often not even referring to Garp at all.
So we see chapters titled The World According to Marcus Aurelius and The World According to Bensenhaver. In the first case, it's used in reference Meditations by Marcus Aurelius, which serves as inspiration to Garp while he lives in Vienna and then during the aftermath of the car accident. Bensenhaver, on the other hand, is a novel written by Garp himself in direct response to that accident.
See, the phrase—The World According to _____—is used to express the power of writing. The first time we see it in action, it's in reference to Garp's difficulty expressing his own worldview. And all writing, ultimately, can be seen as an expression of a personal worldview.
As an interesting note, The World According to Garp didn't receive its iconic title until pretty late in the game—Irving had been planning on titling it Lunacy and Sorrow, which is another phrase that pops up quite a bit. In all honesty, it's not that big of a change, since the world according to Garp, as you well know, is full of lunacy and sorrow.