I have a recipe for GF bread that calls for baking in a loaf pan size: 7.5" x 3.5".
Would this recipe work in a pan that is "8.5 x 4.5"?
I bet it'll work just fine. Expect your loaf to be thinner overall when using the larger pan. Gluten-free bread tends to be a sticker bread dough than gluten-based breads and you can get a higher loaf out of it by filling the loaf pan fuller than you would normally. This is probably why the recipe calls for a smaller loaf pan--so it can get a good solid sandwich-type height.
Yes, I agree with Syronai, that pan size will be fine. It will just not be as high as a smaller pan.
Keep an eye on it though, it probably won't have to bake for as long.