CONFIG_MACH_GTWX5715 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Gemtek WX5715 (Linksys WRV54G)

modulename: gtwx5715-setup.ko

configname: CONFIG_MACH_GTWX5715

Linux Kernel Configuration
└─>Intel IXP4xx Implementation Options
└─>Gemtek WX5715 (Linksys WRV54G)
In linux kernel since version 2.6.12  
This board is currently inside the Linksys WRV54G Gateways.

IXP425 - 266mhz
32mb SDRAM
8mb Flash
miniPCI slot 0 does not have a card connector soldered to the board
miniPCI slot 1 has an ISL3880 802.11g card (Prism54)
npe0 is connected to a Kendin KS8995M Switch (4 ports)
npe1 is the "wan" port
"Console" UART is available on J11 as console
"High Speed" UART is n/c (as far as I can tell)
20 Pin ARM/Xscale JTAG interface on J2

source code: