Canada ETA: Who May Apply:

Lawful permanent residents of the U.S. (green card holders) need an Electronic Travel Authorization (ETA) to fly to or transit through a Canadian airport.
You must present a valid Green Card and a valid passport at airport check-in.

The following nationals do not need an ETA when traveling by land (bus/train) or sea (boat/cruise ship), however, you will need an ETA to board a flight (traveling by air):