Is this some darn Canadian acronym that everybody under the Maple Leaf understands?
Everyone from Ontario-—GTA is Greater Toronto Area, also known as the “905” from the area code that rings the city, to distinguish it from Toronto proper “416.”. Outside Ontario, I believe the GTA is normally used understanding it includes Toronto as well, though within Ontario I would say that is likely a minority usage, and definitely not how it is generally used within the 416.
As for everyone in Canada knowing it-—most people in Canada outside of the GTA are aware that the GTA thinks itself the centre of Ontario, Canada, and the Universe, and that this attitude radiates out from the 416 territory. Outside of Ontario, the arrogant attitude is (to some extent rightly) seen as extending to all of Ontario, though generally recognized that it is centred in Toronto.
For a good perspective, listen to the “Toronto Song” by the Edmonton band “Three Dead Trolls in a Baggie”
https://www.youtube.com/watch?v=_CF65H7j5Os
The best and most insightful of many, many slams against Toronto in the song is “they think they’re New York.”
Having grown up in the western U.S. but gone to school in the east, and having experienced the east of both countries to a great extent and knowing many western Canadians, I would say that hits the nail on the head.
Greater Trudeau Asylum.
GTA = Greater Toronto Area