The point is that you do not want to show that you've established common law at any time before your GF landed in Canada.
If that was the case (which it appears not to be) then she would have had to have declared you as a common law partner on her own application. Failure to have done so if you were common law would be grounds to exclude her from ever sponsoring you, and could even lead to the charge of misrepresentation on her own application.
This is not the case though, because from everything you have said here you are not common law. However, because you will be filling in background information that includes addresses, you will have to write a letter of explanation, or provide information that shows that when you lived in the same building, you were not, in fact, living together as a couple. You will have to explain to the officer that this was shared accommodation based on employment but that you had separate living areas. You would be providing this to show that you are NOT common law because of what I mentioned in the paragraph above, and what others have also said in this thread.
Now that your GF is a PR, you could still become common law by living together anywhere in the world as a couple for at least 12 months. So, theoretically, she could live together with you for a year in which you become common law, return to Canada, and sponsor you. She would have to be mindful that she is able to meet the Residency Obligation of 730 days in a rolling 5-year period to be able to maintain her PR status, but that's one of the paths you could take.
The other, less time-consuming, more straight-forward, and more obvious path is that your GF sponsors you as a conjugal partner.