An ETA will be needed to travel to Canada. In this situation, if you hold a US Green Card then you qualify for an ETA (Electronic Travel Authorization). The Government of Canada requires you to obtain an ETA to enter Canada by air. The ETA covers multiple entries and is valid for up to five years (or the life of your passport). Proof of travel is not required. If you are traveling in the near term or expect to visit Canada multiple times we recommend you secure your ETA now.
Canada ETA's are required for all foreign travelers entering Canada by AIR that are USA Permanent Residents (must be a Green Card holder) or any foreign passport holder who is normally visa exempt (whether or not they have USA Green Card)