http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=593&top=16
Q: I have U.S. residency (Green Card). Do I need a visa or an eTA to visit Canada or can I use my Green Card?
A: As a U.S. Green Card holder, you do not need a visitor visa to travel to Canada. However, you are expected to have an eTA if you plan to fly to or transit through Canada.
When flying to Canada, you will need to present:
- proof of status in the U.S. (such as a valid Green Card), and
- your valid passport that you used to apply for your eTA.
The border services officer will verify your eTA when you arrive in Canada.
When travelling by land or sea directly from the U.S., you will only need to provide proof of your U.S. lawful permanent resident status (such as your Green Card).
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=593&top=16
