Yikes. If you break down the clip, you'll see Reimer as the very top of his crease as Gionta cuts across. When Gionta makes contact with his back to Reimer's mask, Reimer is preparing to make a rebound save. It's a bit of a muddle in front, as Dion Phaneuf makes contact with Gionta too.
Hopefully, Reimer is OK. The Leafs are due to update after the game. (Glenn Healy on Hockey Night In Canada claimed Reimer embellished the play. Seriously.)
If nothing else, this play tonight proved definitively that Brian Gionta can, in fact, deliver a head shot on another NHL player. Provided that player is on his knees at the time.