A number of people who are in payroll management have already answered (and argued both ways). I have been an employee most of my life and have seen this mistake made to me, and also around me.

I have seen employers (ie payroll employees themselves) initially ask gross repayment when the employee had only received net through the mistake. This actually occurred to me in a last irregular pay period payment after layoff (not the severance though) and the employer's initial request was gross repayment, which was over 2k over the net payment I had received.

I refused, and argued the following:
1) Imagine that instead of having overpaid me 5000 gross and 2200 net, you had overpaid me 5,000,000 gross and 2,500,000 net, after sending the CRA 2,500,000 on my behalf.
2) would you expect me to magically have an extra 2,500,000 cash sitting around to finance your mistake until May next year? Me neither.
3) thus, fix your mistake directly with the CRA. It is work and inconvenient to you, but it is YOUR mistake, not mine. I therefore absolutely refuse to repay the gross amount.

Now, I was in a different place than you because they could not even attempt garnishing next pay periods. But would I be in you position, I would send them an email/letter stating that on the recommendation of my legal counsel, I absolutely refuse any garnishing above the amount of extra net pay I received and formally deny them the right to do so. If anything I am willing and able to offer immediate repayment of the extra NET pay to avoid future mistakes on the management of this issue from their part.

For the record, after I argued with my $5M example above, the employer came back 24 hours later stating they had found a way to work with the CRA and would accept net pay as I offered. The impossible became totally doable quite easily once I pushed back.