In the aftermath of Gatsby's lavish party in chapter 3, Owl Eyes gets involved in a drink-fueled car crash. Thankfully, it's not fatal, but the brand-new coupe is in a right mess; one of its wheels has come off. A curious crowd of onlookers begins to form around the wrecked auto:
Half a dozen fingers pointed at the amputated wheel (p.55).
This is an example of personification because something that isn't human—a wheel—has been given human qualities. Amputation is something normally associated with the surgical removal of a limb. The juxtaposition of "fingers" and "amputated" is especially noticeable here.
Earlier on in the scene we're given another example of personification (emphasis mine):
Blinded by the glare of the headlights and confused by the incessant groaning of the horns, the apparition stood swaying for a moment before he perceived the man in the duster.
Horns don't groan, of course, but humans do. Fitzgerald's use of personification is highly appropriate here, as it shows the impatience of the other party guests in their cars as they find themselves blocked in the road by Owl Eyes's auto wreck.