If it's during free dining, it might be a bit of a wait for a table. If you are on the deluxe plan, it will take extra time for the appetizer also.
Last fall when we were there in September we had over half and hour wait and dinner took a while (not the best service or food..hair on my marshmallow with my fondue).
If you are just going to see the fireworks from the boat dock at the GF, you would probly be fine. If you have time, I would take the monorail over to the beach at the poly...MUCH better view. It's really gonna depend on how backed up they are I think. They dont' seem to be the most organized restaurant.
If you are going to the party, I strongly recommend being in the park during the party..there's a lot to do and see and the GF cafe is certainly no comparison. Either eat earlier if possible or get something in the park.
If you are rushing to the fireworks, you will also be missing the castle light up ceremony with the fairy goodmother also.