The Colts had just taken a lead on Adam Vinatieri's 37-yard field goal with 56 seconds left, but the Jaguars (1-2) came right back.
On the decisive play, Gabbert hit Shorts with a pass, and he ran past the Colts defense into the end zone for a touchdown, followed by an unsuccessful 2-point conversion attempt with 45 seconds remaining.
The Colts (1-2) then staged a last-chance drive, and Andrew Luck got Indianapolis to the Jacksonville 26 with a 36-yard strike to T.Y. Hilton.
But on the last play of game his pass into the end zone was batted away.
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend
Pot vending machine to debut