First of all, in order to keep the bottom of the galette from getting soggy, it's important not to roll the pastry too thin. So, try not to roll the puff pastry square larger than 13 inches. Also, it's helpful to control the moisture in the filling. Keeping the layer of apples relatively thin and even helps with this. As does the thin layer of flour added to the pastry, which absorbs some of that moisture from the apples as they bake. Finally, it's really important to make sure to bake the apple galette for long enough. The juices in the galette should be rapidly bubbling, and, the underside of the galette should be a deep golden brown (you can peek at this by lifting one side of the galette up with a spatula!).