Originally Posted by
Aussienarelle
It has nothing to do with your green card which allows you entry to the USA.
OP, as a green card holder (and assuming non-Canadian citizen/permanent resident), requires eTA to
enter Canada.
Lawful permanent residents of the U.S. need an eTA to fly to or transit through a Canadian airport. They must present a valid Green Card and a valid passport at check-in.
I'm not eligible for eTA so I can't help OP or comment on what UA requires.