What song is playing during the first encounter with Granzeboma?
It is from the second OST, track 6, titled BafBaf! そんなに燃えるのが…好きかい ("BafBaf! Sonna ni Moeru no ga...Suki Kai?", "BafBaf! Being Fired Up Like This... Don't You Like It?"). 

The scene in the question starts playing from around 02:28 into the track.

The song was also featured in Gurren Lagann Parallel Works music video #2.