Up until last year, drivers could race back to the caution flag (waving at start/finish). The first place car was declared at the start/finish line. Last year, when Dale Jarrett's car, for whatever reason, was left sitting in the middle of traffic heading out of turn 4 toward the start/finish drivers buzzed right by him. It was very dangerous, so NASCAR finally implented the current rule that the positions are locked when the caution is brought out.
Gordon, according to NASCAR. was leading the Talledega race today when the caution came out for #25 Brian Vickers' spin. It also fell within 5 laps of the finish, so NASCAR didn't wave the red flag to stop the race. There wouldn't have been enough time to (safely) restart the race under green flag conditions.