Gano's kick made up for a 47-yard miss that could have won it for the Redskins (5-5) at the end of regulation.
Donovan McNabb drove Washington from its own 13-yard line to set up the winning overtime kick. The key play came when a replay review reversed a would-be Tennessee interception near midfield.
The Titans (5-5) were also penalized for roughing the passer on the decisive drive.
McNabb went 30-of-50 through the air for 376 yards, one TD and one interception for Washington. Santana Moss made six grabs for 106 yards and a score for the victors.
Vince Young passed for 165 yards and Chris Johnson racked up 130 yards on 21 carries in Tennessee's loss.