Girls Dead Monster is a fictional all-girls rock band from the 2010 anime Angel Beats. In-universe, they work for the “Afterlife War Front” student organization as a group meant to perform music for the students as a distraction to allow the rest of the Front to perform their operations.
Their music is quite popular both in-universe and out, spawning an album and various singles that almost all reached the top 10 of Japan’s Oricon charts.