Not quite. Although 99% of the time, the duties are purely ceremonial, there are rare circumstances in which the Governor General may exercise a great deal of power. The Governor General has certain reserve powers to be used only in situations of constitutional crisis (or on the advice of the Prime Minister).
These powers have been used at least four times in Canadian history - in 1896 and 1925. In 1896 a Prime Minister refused to resign after losing an election, and the Governor General was forced to dismiss him from office. In 1925, the Prime Minister asked the Governor General to dissolve Parliament so he would not have to face a motion of no confidence, and the Governor General refused.
A good government will do its best to prevent a situation where the Governor General is ever forced to consider using the powers (except on the advice of the Prime Minister). It's when you have a bad government seeking to abuse its authority that they become relevant.
Thanks for the history lesson, very interesting.