You're probably thinking of Zombies, Run!, which was an incredibly popular game that went viral over a decade ago. IIRC it did use GPS, but more for tracking your speed, distance, and overall movement. I don’t think it used your precise location to determine the proximity of zombies, though. Was a pretty fun game.

https://en.wikipedia.org/wiki/Zombies,_Run!

A zombie game properly paired a decent pair of "AR glasses" (that worked well outdoors) would be great fun to just head over to the nearest unoccupied soccer field and run for hours tho.