Lockers are not available.
From the GTAA website:
Coat and luggage storage facilities are available pre-security at The Travel Store. For more information, contact one of the following locations:
* Terminal 1, Level 3: (416) 776-1313
* Terminal 1, Level 3: (416) 776-2247
* Terminal 3, Departures Level: (905) 612-6677