The G7 is an informal group, consisting of the most developed countries of the western world ("The First World"). This organization, joined with the United Nations and some other countries, including Russia, formed GDI sometimes prior to the First Tiberium War, that was waged against the shadowy superpower, the Brotherhood of Nod around the Earth in different locations.
The current countries, who founded the G7, are:
Note that while Russia is a GDI member state during the First Tiberium War, the real world G8 did not exist until 1997, two years after the release of Tiberian Dawn.