From what can be seen in the diagram above, protocol encapsulation (not specifically GRE) breaks the layering order in the OSI model terms. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.
IP as a delivery protocol
GRE packets which are encapsulated within IP will use IP protocol type 47.
A GRE packet header structure is represented in the diagram below. [Latest RFC 2784 && RFC 2890]