The GG signs off on legislation, to put her role as basically as possible. She does this as the Queen's representative, and in practice, this is her primary and really her only role in government.
Legislation is passed by the House of Commons and Senate, but is not in force until it is "assented to" by Her Majesty, or in our case by the GG acting on her behalf.
For instance, the Employment Insurance Act starts out (as do all federal statutes):
http://www.canlii.org/ca/sta/e-5.6/part50270.htmlAssented to 20th June, 1996
Her Majesty, by and with the advice and consent of the Senate and House of Commons of Canada, enacts as follows: ...
Her second role is to enforce the Constitution and constitutional convention in relation to the existence or continuance of a government or a Parliament, itself, in the event that a government or a Parliament ever attempts to gain or remain in power contrary to the Constitution or constitutional convention.
If the government were defeated on its budget, for instance, and refused to resign, the GG could step in, relying on Canadian constitutional convention, and dissolve Parliament and call an election.
There is no intent, and certainly no likelihood, that the GG would ever step in and dissolve a Parliament -- or do anything else, like refuse to assent to legislation -- unless Parliament itself were the one acting contrary to the basic constitutional values of the country.
In the case of legislation, the courts perform that role: they can strike down legislation that is contrary to the constitution, which is the formal expression of the society's values. But in the case of a Parliament (i.e., one would think, the governing party) refusing to seek an election, either because its time was up or because the governing party had lost a confidence vote, the courts have no jurisdiction. Rather than have a vacuum in which force might be the deciding factor, we have a head of state to do the job.
This is why the nonsense that is all too regularly heard from the cheap seats, about "subjects" and suchlike, is just, well, nonsense.
The role of the monarch (in our case, viceroy) in a constitutional monarchy is to uphold the constitution, and that's all.
Well, apart from handing out awards and hosting dinners and going to weddings and funerals -- all as the official representative of *us*, i.e. all of us, which the PM, as a politician, isn't.