everyone interprets the bible in their own way, but for the most part it seems like trans is a difficult thing for a lot of religious people to accept (moreso than homoloveuality). still, if your grandpa is a good guy, im sure he wont hate you for it even if he doesnt agree with it. he might try to preach some stuff to you, but i mean it was probably gonna happen at some point anyways so just tough it out. family is still family at the end of the day, im sure your gpa respects that