Additional on Gabby Williams....In order to be a great rebounder, you have to have excellent "timing", in addition to having "hops"(Gabby has hops). The two go hand in hand.
You can be a good rebounder if you're lacking one of these attributes, but if you're blessed with both, along with a nose for the ball (which can't be taught, it's a gift), you'll be one heck of a rebounder.
A basketball player that exemplified all of these traits was DENNIS RODMAN. Another player was Darrell Griffith (Utah Jazz), also known by his nickname Dr. Dunkenstein. Griffith was only 6'4", but was widely regarded for his leaping ability, and his monstrous dunks. I would mention Michael Jordan here, but he's in a whole different class. He (along with Julius "Dr. J" Erving) personified "hops", and exhibiting "hang time".