From Command and Conquer Wiki

The Group of Eight, or G8, is a forum for the nations of Canada, France, Germany, Italy, Japan, Russia, the United Kingdom, and the United States. Prior to the arrival of Tiberium, these nations controlled over half of the world's wealth. As prominent members of the UN, the G8 were the main source of funding for the nascent GDI.

With the arrival of Tiberium, the Brotherhood of Nod was able to use it's advancements in tiberium harvesting to gain almost half of the world's tiberium reserves, granting it enough finances to rival the G8.

As tiberium spread across the globe, and national boundries became less important than the boundries between zones, the G8 largely faded into irrelevance.