CONFIG_NF_CT_PROTO_GRE (not configurable)

configname: CONFIG_NF_CT_PROTO_GRE

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Network packet filtering framework (Netfilter)
└─>Core Netfilter Configuration
└─>NF_CT_PROTO_GRE
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  

is depended by
CONFIG_NF_CONNTRACK_PPTP