CONFIG_GAMECUBE is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Nintendo-GameCube

modulename: gamecube.ko

configname: CONFIG_GAMECUBE

Linux Kernel Configuration
└─>Nintendo-GameCube
In linux kernel since version 2.6.33 (release Date: 2010-02-24)  
Select GAMECUBE if configuring for the Nintendo GameCube.
More information at: <http://gc-linux.sourceforge.net/>

source code:
arch/powerpc/platforms/embedded6xx/gamecube.c


is selected by
CONFIG_RTC_DRV_GAMECUBE