Sad enough I still use the 6 boroughs as reference points to Understand retail capability of an area. Calling it the "GTA" is too broad as someone from etobicoke is probably not going to drive to east york to shop
The populations in 2001 are available on Wiki and the statscan website.
In 2006 StatsCan no longer recognized the boroughs, though IIRC "Old" Toronto increased a few percent (~700k), Scarborough (~600k) and North York (~540k) by 5% or so, and York (150k), East York (115k), and Etobicoke(340k) declined slightly. The individual census tract profiles are available and one could determine the population of the boroughs with a bit of determination.