CONFIG_ARCH_GUMSTIX is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Gumstix XScale boards

modulename: gumstix.ko

configname: CONFIG_ARCH_GUMSTIX

Linux Kernel Configuration
└─>Intel PXA2xx/PXA3xx Implementations
└─>Gumstix XScale boards
In linux kernel since version 2.6.26 (release Date: 2008-07-13)  
Say Y here if you intend to run this kernel on a
Gumstix Full Function Minature Computer.

source code:
arch/arm/mach-pxa/gumstix.c
selects

CONFIG_PXA25x

depends
CONFIG_ARCH_PXA |
NOT | CONFIG_ARCH_PXA_V7

is selected by
CONFIG_MACH_GUMSTIX_F