Gado, who spent parts of the last two seasons with the St. Louis Rams, scored from four yards out twice in a span of less than three minutes in the third quarter.
Last week, the Titans waived Johnson, who underwent surgery to repair a dislocated right ankle.
Chris Johnson scored on an eight-yard run in the opening quarter. Vince Young started for the Titans and completed 9-of-13 passes for 128 yards. Kerry Collins went 6-of-12 for 66 yards.
Matt Leinart led the Cardinals under center for the first three offensive series and ended 4-of-6 for 28 yards. Derek Anderson replaced Leinart and connected on 11-of-19 throws for 105 yards. Max Hall threw a 22-yard scoring pass to Max Komar midway through the final quarter.
After Gado scored his first touchdown with five minutes left in the third, the Titans got the ball back at the Arizona 15 when punter Ben Graham fumbled the snap. Gado scored three plays later for a 24-3 lead.