I was unable to find a precompiled GPIO module for the Atheros build. So I created a new Openwrt Package which is based on Ticket #1172. The module itself was created by olg and is hosted here.

On this site you can find the GPIO pinout:

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Pinout Fonera
——————-
GPIO    Description
0       TP3
1       pin 5 of SW1
2       WLAN LED
3       pin 1 of SW1
4       pin 2 of SW1
5       RESET (!)
6       RESET button
7       pin 6 of SW1

They also demonstrate how to use it:

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echo WLAN LED on
echo 1 > /proc/gpio/2_dir
echo 1 > /proc/gpio/2_out
echo WLAN LED off
echo 1 > /proc/gpio/2_dir
echo 0 > /proc/gpio/2_out

Download the source or the (soon outdated) binary package (Kernel v2.6.23.1).