I don't know how many people outside of Toronto (or Canada) know what GTA means. (I know that I didn't). Abbreviations that make sense locally rarely resonate globally.
As someone that grew up a few hours from Toronto, I'd be confident in saying that the GTA is in the common vernacular for most folks in Southern Ontario. I agree that those outside of that area (in Canada at least) wouldn't, since from outside it looks like it's all Toronto.