Can airliners be considered as GA? In a Swiss001 video, he said that the Twin Otter was a general aviation (GA) plane. But then in another video, he said that the Twin Otter was an airliner. Obviously, he's not a credible source, but it does make me wonder: Can airliners be GA aircraft planes at the same time? 

(I mean, some airlines do operate King Airs to remote destinations)
GA refers to why you fly your plane, not what kind of plane it is. For example, John Travolta has (had?) his own personal Boeing 707, which is the first generation of jet airliner from Boeing. When he flew it, it was not under scheduled service, so it would be classified as a "general aviation" flight.