My Iphone 12 Serial number starts with G. What does it mean?

Nothing, absolutely nothing.

Do you have an issue with your iPhone?

From 2021 or so, Apple serial numbers are an eight-to-fourteen alphanumeric string.

Prior to that change, Apple serial numbers used a more predictable format.

Your serial number starts with G.

You are probably thinking about the model number, where the initial letter of that can have a particular meaning.

But you bought it used and it seemingly wasn’t locked or bricked or blacklisted, so you’re ahead of the average used-device purchase that gets discussed around here.