Arcane Comet + GP Ultimate.
This is a sample build I made for GP, might be tweaked or a little off, but one thing for sure, The tenacity one is GREAT and so is the extra gold on takedowns. Pare this with Arcane Comet oh yah bb, hes back.
And then The Ultimate Hat is great synergy with the above mentioned and so is Transcendence.
Scorch because it synergizes with his Trial By Fire and its the only really viable one on the aslt row. Arcane Comet is a must for GP. It gives him the global executing power thunderlords used to give. Yay, ty rito.