E! is reporting that GOSSIP GIRL star Chace Crawford will guest star on the 100th episode of GLEE. He will play Quinn's love interest, Biff.The 100th episode will center on Jane Lynch's principal Sue Sylvester, who after years of disdain for the New Directions and Matthew Morrison's Will Schuester, finally succeeds in ending the glee club in Lima. Many original cast members -- Heather Morris (Brittany), Harry Shum Jr. (Mike), Mark Salling (Puck), Amber Riley (Mercedes), Dianna Agron (Quinn) and Kristin Chenoweth (April), the latter of whom won an Emmy for her guest turn -- will return to say their final goodbyes to Will and each other in an episode that is said to be very moving.