How can forgiveness be sought from a deceased grandmother after an act occurred that possibly offended her, and it was not possible to ask for her pardon during her lifetime?

It is no longer possible to ask your grandmother for forgiveness after her death, and this underscores the importance of the Prophet's ﷺ hadith: "Whoever has wronged his brother, concerning his honor or anything else, let him seek his forgiveness today before there will be neither dinar nor dirham." For the wronged person will take from the good deeds of the wrongdoer on the Day of Judgment, or the wrongdoer will have some of the wronged person's sins cast upon him. If your repentance to Allah is sincere and you frequently supplicate for your grandmother, she may forgive you, and Allah forgives everyone.