Key revealed after the meeting that Trade Minister Todd McClay is heading to Saudi Arabia next week.
Key said it was a good meeting with the Saudis: "We didn't hold our punches."
"I think we are getting closer."
He said he was quite optimistic the deal would come off.
"They are quite aware of what they need to do. They are considering a package of things at the moment, so yeah, we are getting closer."
The GCC is made up of Saudia Arabia, United Arab Emirates, Qatar, Kuwait, Oman and Bahrain but Saudi Arabia is the heavyweight.
Key also had bilateral meetings with the President of the European Council, Donald Tusk and the Prime Minister of Pakistan, Muhammad Nawaz Sharif.