I'm not talking about a chisel.
Think about how great-grandpa's information got on his tombstone. Someone thought they knew when he was born and so they told the guy who made the stone. Probably that same person provided the death information. If the stone was put up years after great-grandpa died, it is possible the stone has the wrong date of death. And the date of birth could be wrong as well.
Tombstones are usually primary sources for the date of death, unless you can clearly tell it was erected years after the person died. The main thing is to transcribe it exactly as it is written. Your discussion of why you think it is wrong, right, etc. should be done in your notes.