The idea that Nebuchadnezzar's seven years of madness represented the 2520 years of the "Gentile Times" was part of Russell's belief system. But there was a huge difference, because Russell did not believe that the tree which was cut down and restrained from growth in the vision, represented the idea of God's kingdom being in abeyance. It stood for, first Nebuchadnezzar and, second, humanity in general. So the end of the Gentile times was to be marked by humanity coming to their senses (like Neb did) and acknowledging God's sovereignty. So, Tree = mad King Neb = degraded humanity. A fairish consistency.
In the post-Russell make-over the end of the GT is marked by the setting up of the Kingdom of God in heaven. So now, Tree = mad King Neb = dormant Kingdom of God.
In other words Neb, the king of Babylon (which represents Satan's Org) stands for the Kingdom of God. Does that seem just a little bit silly? But silly is what Joe Rutherford and his minders were good at.
Rob Crompton