Others use it differently, but I think he was using "GP" to mean "grandparent", as in the comment you would get to if you clicked on "parent" twice starting at

Others use it differently, but I think he was using "GP" to mean "grandparent", as in the comment you would get to if you clicked on "parent" twice starting at his comment. In that system, you would be the "OP", not the "GP", although this too is not fully standardized.