Maybe I could flesh out the situation here.
When you want a citizen to appear before the committee and they refuse to attend, you have the optioning of summoning the witness. If they don't attend, you can go to the House and seek a contempt.
In my view, the House of Commons can't hold the Governor General in contempt any more than it can hold the Queen in contempt. Constitutionally, the Governor General is not accountable to the House. The Queen is not accountable to the House. Notwithstanding that by tradition since the times of King Charles the Queen can't walk into the House of Commons, she can't be called in, in the sense that she is the monarch and the rest of us exist under that. That's the theory.
So if you can't summon the official in their official capacity or hold them in contempt, it follows that they have an obligation to attend in their official capacity. The tricky part comes in terms of the former office holder. While that person can be summoned, even held in contempt, once you get into questions about their official duties, you're in effect making them accountable to the House. That could be a problem.