The quarterback hit up a gym in New York City on Sunday to get his pump on ... just in case that phone rings from an NFL team.

Kap was sporting Nike head to toe -- the company has not only stuck by Colin, but has featured him in ad campaigns over the years.

Our Kap sources tell us Colin's been hitting the gym DAILY and working out with a QB coach regularly ever since he last stepped foot on an NFL field back in 2016.

Now, with Andrew Luck announcing his retirement on Saturday ... and other teams trying to figure out their quarterback situations, seems the timing could finally be right for a Colin Kaepernick comeback.

In fact, Colin's pal, Eric Reid, recently said he'd like to see Kap get a shot with the Carolina Panthers if (and only IF) Cam Newton is too injured to play after suffering a foot injury during a preseason game.

Reid stressed that he supports Cam and is rooting for Cam -- but says Kap would be an excellent Plan B.

Kaepernick has made it clear to friends and fans that he wants to keep playing football -- and recently debuted a counter on his social media pages showing the days he's been out of work.