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Sorry to hear about the frustration. It does not seem like the CRA can tell you anything, if they don't have any data.
I would suggest continuing to bug GM about it, they should be able to research the data transmittal to the CRA to say whether they got a confirmation of receipt, or nothing.
Rather than GM telling you to go call the CRA, GM should be researching to confirm that the CRA communication went through.
Good luck!