Gifting Stocks to a dual citizens of Canada and US

https://www.taxtips.ca/personaltax/giftsandinheritances.htm

There is no "gift tax" in Canada. Any resident of Canada who receives a gift or inheritance of any amount from almost any source (except from an employer) will not have to include this in their income.

Your niece won't owe anything.

However, if capital property (e.g. real estate, investments) is given as a gift, the person who has given the gift will be deemed to have sold the capital property at fair market value (FMV), and will have to pay tax on any resulting capital gain.

And this Canadian law does not apply to you, an American citizen living in America.