Great question — answering as the UphillTech team:
If travelling by land, only the green card is required (no passport or other travel document is needed). If travelling by air, you need the green card and a passport or citizenship certificate. Nowadays, however, no need for an eTA. SOURCE: CBSA
Full answer by André Eklöf: https://travel.stackexchange.com/a/87368 (CC BY-SA 3.0).
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