at the time of GST registration, if a person doesn't having address proof documents since the applicant's father died 20 years back and property is still in the name of father.
It is advisable to change the name in the Electricity Bill since continuing the name of a deceased consumer is not a legal practice. You can change it to your name or in the name of Spouse ( your Mother's name ) and take a No Objection Certificate ( NOC ) from your Mother and provide the same in the GST Registration application...
This is my personal advise. Also consult with other members of the forum for better suggestions...