ETA or Arrivecan ?
According to the requirements
Travellers who are exempt from the eTA requirement.
U.S. citizens are exempt from the eTA requirement and must carry proper identification such as a valid U.S. passport.
Lawful permanent residents of the United States who hold valid status in the U.S. are exempt from the eTA requirement.
They must carry official proof of status (or an equivalent status document) and a valid passport from their country of nationality (or an equivalent travel document).
See the complete list of documents for lawful permanent residents of the U.S.
https://www.canada.ca/en/immigration-refugees-citizenship/services/visit-canada/eta/facts.html