Serial number beginning with a G indicates that is out of Foxconn's new factor(ies) in China. Nothing wrong with that.
If an iPhone is a knockoff or a fake then the Serial number and the model number can be fake as well. The real way to determine whether the iPhone is real or fake is to really be able to compare it physically to the real one. OR know the real iPhone well enough to distinguish it from the fake mode.
In many words, I am trying to inform you that the decision to whether buy it or not is entirely yours.
Axel F.