GGNMOS ideally works when IO zaps to VSS or VSS zaps to IO. GGPMOS ideally works when IO zaps to VDD or VDD zaps to IO. So, both should be placed on chip. GGNMOS should be more effective as it's mobility is higher for electrons than holes. However, as for this, gate length got to be thicker than PMOS gate length. GGPMOS can be of more current uniform.
I would say, GGPMOS or GGNMOS are both NOT effective. You should use resistive GGNMOS or resistive PMOS or pseudo grounded PMOS/NMOS as your ESD diode. I once implementted GGNMOS/GGPMOS and they CANNOT withstand over 2000V HBM, afterwards, I changed to resistive NMOS/PMOS and can withstand over 2000V, but still not much, at least may meet your requirement