I've been told different about other country's. I always thought that entering another country exept your country or the US with a US P. Residency Card you would need a passport but only the permanent residency card to return to the US.
This is not true. Canada does not require a passport for U.S. permanent residents entering Canada directly from the U.S. And the U.S. does not require a passport for U.S. permanent residents returning to the U.S.
It's also good to note that even when flying to and from Canada, a passport isn't required for an LPR. However, some airlines could cause trouble. So, do this at your own risk. By land and sea, using just the GC is no problem at all.