default or selected kernelversion does not have config value CONFIG_RXGK.
Result is shown for kernelversion 6.16-rc7

RxRPC GSSAPI security

configname: CONFIG_RXGK

Linux Kernel Configuration
└─>Networking support
└─>RxRPC GSSAPI security
In linux kernel since version 6.1.145 (release Date: 2025-07-14)  
Provide the GSSAPI-based RxGK security class for AFS. Keys are added
with add_key().

See Documentation/networking/rxrpc.rst.