Here's video proof Josh Gordon is taking this whole "NFL comeback" thing seriously ... he's in a hardcore training program in a famed L.A. gym with a famous L.A. trainer.
The Cleveland Browns wide receiver -- who applied for reinstatement into the NFL after a drug suspension -- has been working out with Jay Glazer at Unbreakable Performance Center in L.A. and we gotta say ... he doesn't look rusty at all.
Sources close to the Gordon say Josh has been keeping himself in insane shape during his suspension -- and has been laser focused at getting his career back. As one source put it, "He's all in."
Peep the vid -- dude looks cat quick and locked in ... not out of the woods yet, but it's a start.