Yes, of course you can ask a priest to say a Mass for the repose of your grandmother’s soul. The soul of any deceased person can be prayed for with a Mass, except those whom we know are in heaven. Those we know are in heaven are the saints and Masses in their honor are said on their feast days.
The Catholic Answers staff joins in your prayer for the repose of your grandmother’s soul and we ask our forum participants to pray for her and for your family. God bless.