Gylfaginning serves as the primary narrative core of the Prose Edda, laying out the full mythological framework from creation to destruction in a coherent, continuous account. By contrast, Skáldskaparmál functions as a supplementary collection. It expands on individual myths, anecdotes, and figures, mainly to illustrate poetic language and the use of kennings. Together, they form a layered system: Gylfaginning provides the structure, while Skáldskaparmál fills in detail and shows how that material is used in skaldic tradition.
I consider the Prose Edda to be a secondary source while the Poetic Edda is the primary source.