Unfortunately, once the juices have already leaked out there is really nothing you can do. To prevent this, make sure there are no gaps or cracks in the edges of the crust you've folded up and try to give the galette enough time to cool and set up a bit when it comes out of the oven. It can also be helpful to bake the galette on a rimmed baking sheet so that if juices to leak out, the sheet pan will catch them all.