I borrowed from the HELOC and gift it to my spouse for her down payment of a rental property that is fully under her name. Can I deduct interest?

No, you can't claim interest payments for money that you gifted your wife.

Thanks for the answer Susan. 🙂

For my future reference, how about spousal loan instead of gift? 

Would the interest from HELOC be tax deductible as they are lent to my wife to invest and I receive interest payment from her based on the CRA-prescribed interest rate? 

Yes, if you make a loan to your wife and it meets the CRA's requirements, you can claim expenses incurred to make that loan, which can include your HELOC interest. But  you would need to consult with a tax law expert if you wanted to make a loan to your wife that doesn't attract the CRA's auditors.