You can find an appropriate config file config.gz on your Kobo in the /proc directory. This is the config that was used for building the kernel. Just extract and copy it into your linux source directory.
Then you have to do a make menuconfig to select the g_ether module for building (<M> option). Now, you can build the modules with make modules command.
May I ask what tool-chain you are going to use? With the CodeSourcery chain I had no luck, neither on a Windows nor on a Linux box. Now I am using the gnu-arm chain in an Ubuntu environment.