If Gawyn attacks, then he'd try to stop Gawyn. Especially if Galad comes to believe the prophecies of the Dragon. So far he's willing to fight with the DR, but hasn't (from his POV's at least) made the leap to realising he's essential to the Light.

Unless the 'everyone not a red-shirt gets to live' changes, it's zero chance of happening, but Galad being forced to kill Gawyn to save Rand would be up there as one of the top moments of the series. Now that'd make a great the Dragon shall know the anger of the Amyrlin fulfillment. Egwene finds out Rand let Galad kill Gawyn and goes off her tree.