CONFIG_GDBSTUB is not available for thedefaultarchitecture x86.
Result is shown for architecture mn10300

Remote GDB kernel debugging

modulename: gdb-stub.ko

configname: CONFIG_GDBSTUB

Linux Kernel Configuration
└─>Kernel hacking
└─>Remote GDB kernel debugging
In linux kernel since version 2.6.12  
If you say Y here, it will be possible to remotely debug the kernel
using gdb. This enlarges your kernel ELF image disk size by several
megabytes and requires a machine with more than 16 MB, better 32 MB
RAM to avoid excessive linking time. This is only useful for kernel
hackers. If unsure, say N.

This is deprecated in favour of KGDB and will be removed in a later
version.

source code:

depends
CONFIG_DEBUG_KERNEL
CONFIG_DEPRECATED