Both are completely different beings from different fictional universes with completely different laws and concepts of power and magic. So they are not compareable at all. Harry Potter is a human with magical powers; Gandalf is an immortal divine spirit clothed in a temporary physical form (as are all five individuals called "wizards").
Cannot answer. It's the same as if you asked whether the NCC-1701A Enterprise is stronger than an Imperial Star Destroyer. They're two different worlds, they cannot be compared because they are not meant to be compared.