My research only reveals the answer for US Citizen. I'm US permanent resident. I'm planning to drive to Toronto and fly to Europe and back for two weeks vacation. I've visited Canada before with my green card and passport. I got the Canada eTA for flying back. Do I need anything else?
No, US permanent residents do not need visas to visit Canada, and when crossing in a car don't need an eTA. You will need an eTA for the return flight in (which you have got) so you don't need anything else other than your passport and green card.