Complex sentence: He had previously created items for Gaga, including a costume for the Bad Romance music video for which she won the MTV Music Video Award, and went on to create a hat that she wore to the 53rd Grammy Awards in February 2011.
Split into simpler sentences: He had previously created items for Gaga, including a costume for her music video for "Bad Romance". He went on to create a hat that Gaga wore to the 53rd Grammy Awards in February 2011.