I don't think that there are any buses that run from the TTC anymore. You're only other choice if you are dead set against taking the bus from Poly would be to take either the boat or monorail to the MK and then take a bus to MGM. You are probably better off just dealing with the bus from the Poly as the MK route will easily add 20-30 minutes on to the trip.
Assuming you just make all your connections...
Monorail makes a stop at GF... I have never seen them just stop open the doors, board passengers and go... figure at least 5 minutes till you leave the GF. 5 minutes later you are at the monorail exit... now you have to walk clear across the from of the MK to catch a bus.. maybe another 5 minutes... already you have added 15 minutes on to the trip. If you just miss the monorail and just miss the buss another 15 minutes can easily be added before you are even on your way to MGM.