They cannot be sentenced for the entire incident, they will have to be convicted under some section and death penalty can be then given, death penalty can be given for the below section only
Section 121 of Indian Penal Code, 1860: Waging War against the Government
Section 132 of Indian Penal Code, 1860: Abatement of Mutiny
Section 194 of Indian Penal Code, 1860: Giving or fabricating false evidence leading to procure one's conviction for capital offense.
Section 302 of Indian Penal Code, 1860: Murder
Section 305 of Indian Penal Code, 1860: Abetment of suicide by child or insane person
Section 307 of Indian Penal Code, 1860: Attempt to murder by a life convict, if hurt is caused
Section 396 of Indian Penal Code, 1860: Dacoity with murder
Section 364A of Indian Penal Code, 1860: Kidnapping for ransom
Yes if they would have been given death penalty, it would have been for gangrape + murder. But not for gangrape alone. For only gangrape of Bilkis Bano the maximum punishment could have been life imprisonment.