If you have a U.S. Green Card, you do not need a visitor visa if you want to travel to Canada, Canadian Visa Expert had learned. You will need an eTA, however, if you will be flying to or transiting through Canada.
When you fly through Canada, you will need proof of your U.S. status, such as your Green Card, and a valid passport that you used when you applied for your eTA. When you reach the Canadian border, an officer will verify your eTA.
When you are traveling by sea or by land directly from the United States, you will only need proof that you are a lawful United States permanent resident. A Green Card can be used to show proof. Please note that these rules are subject to change at any time.