GSM's are extremely aggressive toward their own kind. It was inevitable the large one was going to kill off the others. Sorry for your losses. The large one MAY allow the one remaining to live if it decides to mate with it, but there are no guarantees.
I have an established large GSM and introduced a much smaller GSM to my tank in hopes they would become a mated pair. The large one terrorized the small one for a week. I was sure it was going to kill it. But, after about a month she finally allowed the male in the same area and now they are a mated pair. As I understand it, they all start out as males and the largest of the group becomes female. When the female dies off, the next largest male changes to female.
This is in my experience only, others may have had different results.