They've only been together for about 6 months ... and she's 21, while he's 23 -- so breaking up is kinda what you do. However their split comes right on the heels of Squad leader Taylor Swift's with Calvin Harris.
Makes ya think -- is this coordinated?
Neither is saying why they're calling it quits, but a source close to Gigi tells TMZ the writing was on the wall. We're told the fact she never introduced Zayn to her family is "very telling" ... since normally her mom and dad always meet her BFs.