I am a specialist on it.
The Gentile Times Reconsidered is not a great book because he does not reconsider anything. He just tries to prove that 587 is the destruction of Jerusalem. But if he was reconsidering the Gentile Times, he would know that the Gentile times did not start at the destruction of Jerusalem, though that was taught by the organization since Russell. He, like the WTBTS, can't see the forest through the trees.
The Gentile times started when the kings in the line of David lost their sovereignty. That happened prior to the destruction of Jerusalem in 587. The Bible says that Jerusalem was invaded and David was taken captive in the 3rd year of Jehoiakim, whose reign started 22 or 23 years prior to the destruction of Jerusalem. That invasion and Daniel was taken in 607 BCE and that started the Gentile Times because the king in the line of David became a vassel of the Babylonian king and thus was not sovereign anymore. Jehovah's throne was not subserviant to Babylon.
Daniel knew of Jeremiah's prophesy of the 70 years of captivity. He knew when that would end because he knew when that started because he was there when it started. The 70 years ended in 537 and started in 607, when Daniel was taken