Gigi Hadid Boxing In Heels ... And Hot Leather Pants?

Here's supermodel Gigi Hadid ... proving she's one hell of an athlete -- by hitting a heavy bag -- IN HEELS!!!

Hadid hit up Gleason's Gym in Brooklyn -- and despite the fact she was wearing (what appeared to be) leather leggings and chunky heels ... she went all Rocky Balboa anyway -- and looked damn good.

Gigi's been boxing for a while -- and she's picked up a pretty solid overhand right.

#Respect.