The GRE delivery protocol (transport protocol) can be IPv4 or IPv6.
If the GRE delivery protocol is IPv4, then the protocol field in IPv4 header is set to value 47 (GRE). The same value (47 – GRE) is used on the Next Header field if the delivery protocol is IPv6. IPv6 extension headers in the L2oGRE transport IPv6 header are not supported, and packets containing these IPv6 extension headers are dropped.