CONFIG_STX_GP3 is not available for thedefaultarchitecture x86.
Result is shown for architecture ppc

Silicon Turnkey Express GP3

modulename: stx_gp3.ko

configname: CONFIG_STX_GP3

Machine Type
└─>Silicon Turnkey Express GP3
In linux kernel since version 2.6.12  
This option enables support for the Silicon Turnkey Express GP3
board.

source code:
arch/ppc/platforms/85xx/stx_gp3.c