I think neutering a gerbil would be very difficult....There is an extreme caution when neutering rats, and they are five times the size of a gerbil..

It may be that your gerbil Sage hasn't matured yet. Are you sure that you looked at the pics and clearly identified it as a girl? If it is a girl, the little bump should be almost touching the anus. A boy's should be alot farther away, almost on the belly