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Have you heard from the CRA yet?
This is a very difficult situation.
You are going to have to prove that you never got their notice of garnishment.
If you can prove this you will be OK.
And given that your address changed you can likely show you had no idea about these papers, never saw them, were never told about them etc.
If they look like they are going after you then your best next step would be to retain a lawyer to deal with them as they will more likely listen to a lawyer especially if the lawyer threatens a lawsuit if necessary.