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After you did the "modprobe gpu", please verify, that the gpu dev-device was created.
# ls -l /dev/gsl_kmod

The device is chmod 700 by default, so you will only be able to start the xserver as root.

If xf86-video-imx is the only driver installed, you should not need any configs in /etc/X11/xorg.conf.d/

To get the GPU working on this setup:

The kernel-driver is already there (came with the ubuntu-kernel), but not loaded
# modprobe gpu

Install the video-driver and the needed lib
# pacman -S xf86-video-imx libz160

# ldd /usr/lib/libz160.so
The libz160 is linked against the proprietary libkgsl-imx

There is no pkg in the arch repo for that, but you can get it from the ubuntu package:
# wget http://packages.efikamx.info/pool/main/i/imx-graphics/libkgsl-imx_1.0.3-20110310_armel.deb
# ar x libkgsl-imx_1.0.3-20110310_armel.deb
# tar xzvf data.tar.gz
# cp ./usr/lib/libkgsl.so.1.0 /usr/lib/libkgsl.so.1.0
# cd /usr/lib
# ln -s libkgsl.so.1.0 libkgsl.so.1
# ln -s libkgsl.so.1.0 libkgsl.so
# ldconfig