The GoF façade, like a real façade, hides the complexity of implementation, creating another abstraction; it hides a complex and generally separate system (or subsystem) behind a simple interface. For example, the facade of a game can start, update, and pause methods; completely hiding the implementation of the game, but providing a high-level way to interact with it.
On the other hand, the DDD aggregate is a way of defining a has-a relationship between objects that have a stronger correlation than regular links. They can be considered as nodes in the tree of domain objects, and they are usually processed as a whole from the point of view of data exchange.