ask GW to declare war to other nation near his border because in this situation its hard and nearly imposible to ask other nation fight him, bribe him (GW), with city, gpt, lux, strategy item (but not gold) everything u can.
There are one weakness in AI and u can exploit the weakness if he declaring war with other AI, they send almost all of his troops out to attack the enemy, and they rarely rush unit production with gold if there are unwanted attack near their city, the only thing that they can do is send frontline back to defending the city, and it will take time. If u already make him agree declaring war with other AI, wait for 2-3 turn, then attack his empty spot, just say, if the enemy border are at far north, let your troops attacking south. It best to send scout to following where he put his troops and how his war going, so u can decide what is the weakest spot to attack.
But capturing 1-2 city not giving u an instant victory, its only giving u an pre-emptive advantage and an upper hand in opening battle, if u succeed defeating his army in your real battle againts him, dont stop in any cause, keep it until his vanish on map or until he agree to give bunch of his city, gpt and gold to u, u can take it, and sold it to other (his city). It will lead to border conflict that without your effort will wipe him on map and lead to another world war.
About the city states, attack them if only they near your border and can be a threat for your city, if not, dont brother them until you finish your biz with him.
Im not the best player on this forum im not even good comparing to many other, but i really hope u find my suggestion helpful for your situation, try it if u like, and share it. I hope other will contribute to help u out on your situation.