Scomparso means "disappeared" or "vanished." It's a more dramatic and less common word than simply saying something "is not there."
In the context of the song, the singer is frantically searching for his hair gel, and when he can't find it, he exclaims, "È scomparso!" (It has disappeared!). This word choice adds a touch of humor and exaggerated drama to his predicament, making the mundane act of losing hair gel feel like a significant mystery.