Hi Jon,
You're pretty close; it actually runs from north of Leslie and Steeles to John Street (can be confusing to give directions because I think Don Mills becomes Leslie north of Steeles!). I was wandering around GMP last fall with my 100-400 lens when I met this old man who suggested I could do some 'serious fishing' with that lens! When I gave him a funny look he told me he had just seen some large salmon, completely out of the water on a sandbank in the local stream. When I got there they were gone but I returned the next day and was able to get a few shots. There are a few spots where the salmon have to come out of the water and if you can locate the fish upstream(?) from those spots you can go set up and wait for them to pass by.
I really like GMP, it's very close to where I live and compared to many TO parks it's much quieter, even on weekends. In early May there are lots of interesting birds around, along with the great blue herons and kingfishers that seem to be there all year.
