Going to make my first pizza tonight, can a large, thick griddle pan substitute for a pizza stone?
It should work just fine.  Preheat your oven, with the griddle inside, for at least an hour.  Build your pizza on a peel that has been sprinkled with some cornmeal so that it slides off.  If you don't have a peel, use the back of a sheet pan and some parchment paper underneath.  Then slide the pizza and parchment onto your griddle.

If you have a new oven, you may need to go through a preheat/burn-off without food first...check your manual.