Young Gretzky wasn't the fastest guy going in a straight line from 'A' to 'B', but his agility and ability to cut and turn was second to none. He was just way better at that in the early-80s, when not as many forwards could do it. I've really never seen a player (until maybe after the Lock Out when technology improved, yada yada) who could turn on a dime as sharply as Gretzky did, in full control of the puck.
For foot speed, check out this goal from the '83 playoffs, at the old Corral in Calgary:
Gretzky races back on the back-check, strips the Flame player of the puck, plays it through another player's legs in transition to offence (most players would never dare try that in practice, let alone a playoff game), and then races down the ice with the puck. Does this guy look less than fast to anyone?
As to the 'cut-and-turn', this 1985 goal vs. Kelly Hrudey and the Islanders demonstrates it well. The goal itself is kind of cheap as Wayne's shot bounces off an Islander and fools Hrudey, but check out his agility and how easily he shakes Lafontaine with his turn:
Also, it's interesting you say "for such a tall guy". Gretzky was only, what, six feet tall at most? In that Facing Wayne Gretzky book from a few years back, a couple of the players noted that he was taller than they thought he was when they got on the ice with him.