The biggest contributor to a soggy bottom is using filling that has too much water content with not though thickener. This is especially troublesome with a berry galette or juicy tomato slices. I recommend straining out excess juices when possible and adding cornstarch to your filling to thicken the juices.  Soggy bottoms can also happen when the dough is too soft and warm before baking. The dough should be as cold as possible before putting into the hot oven. I recommend chilling the dough after rolling it out and then also placing the galette with the filling in the freezer while the oven preheats.