Different people have different views on this subject.
Some say that Gorbachev was the most important player in ending the Cold War. His programs of glasnost and perestroika changed the Soviet Union in fundamental ways and led to its collapse. From this point of view, Gorbachev's reforms ended the Cold War.
Others argue, though, that Ronald Reagan was more important. They say Reagan's aggressive policies forced the Soviets to spend more on their military than they could afford. This made their economy collapse. From this point of view, Gorbachev was a minor player while Reagan's actions were most important.
Gorbachev's role, then, is a matter of controversy. Perhaps you will want to check to see if your book takes a side on this issue.