Can metronidazole vaginal gel (at night for 5 days) treat cervicitis and bacterial vaginosis at the same time??

Probably OK, but... Bacterial vaginosis is often accompanied by cervicitis. It isn't clear whether BV causes cervicitis or vice versa. Probably you were tested for chlamydia and gonorrhea, which definitely cause cervicitis; double check with your doctor. Assuming you don't have them, BV treatment alone may be sufficient. Oral metronidazole is the usual treatment for BV, usually more effective than the vaginal gel.