THE two sons of late monologist Spalding Gray are also taking to the stage. Forrest Gray, 14, plays lead guitar in Too Busy Being Bored, which recently won a battle of the bands (out of 50) competition at the Knitting Factory. The freshman high-schoolers played a sold-out show in the Hamptons on Saturday night, where more than 100 people were turned away. “Young girls were swooning like they were The Beatles,” a fan told The Post’s Braden Keil. “And Christie Brinkley said it was like being at an early Strokes concert.” Meanwhile, brother Theo, 9, is up for the lead in “Billy Elliot” on Broadway.