In 1975, Gray signed as an undrafted free agent with the Packers. Playing under head coach Bart Starr, Gray became an instant starter in Green Bay and built up a reputation as one of the league's hardest-hitting safeties, though he was just 5-foot-10 and 180 pounds.
"Lompoc's Johnnie Gray, who once earned steak dinners for his aggressiveness on Lompoc's 'F-Troop,' hasn't changed much," a 1975 article in the Record read. "Even though he's risen from third string on the Lompoc varsity to first string on the Green Bay Packer varsity, Gray is is still stinging people.