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

CONFIG_GDBSTUB_ON_TTYSx (not configurable)

modulename: gdb-io-serial.ko

configname: CONFIG_GDBSTUB_ON_TTYSx

Linux Kernel Configuration
└─>Kernel hacking
└─>GDBSTUB_ON_TTYSx
In linux kernel since version 2.6.25 (release Date: 2008-04-16)  
source code:
arch/mn10300/kernel/