I actually would not exist without a great grand father.

Couple points to consider here:

1: Evidence. You have photos or some writings from your grandfather, I assume? Other surviving relatives that knew him and can attest that they met him and spoke with him. And, the low effort evidence, your parents exist, therefore you had two grandfathers and two grandmothers. The only question remains what was his description and name, etc.

2: Relevant need for evidence. Not sure of the best way to phrase that, really. Let's just say that whether you know your grandfather existed or whether you have any knowledge about him does not impact your reality in any way, shape, or form. You really won't ever need to prove your grandfather existed. If a god exists, the level of importance is higher, so the degree of evidence required to prove the existence of that specific god must meet that criteria.

You can't accept any off the cuff simple answer for the existence of a god, it must be reliable and solid evidence. The long-term implications are very high, so the evidence requirements are equally high.

Hope that makes sense, let me know if I need to clarify anything.