Post count: 19

Yes, it is inside Retropie and can be installed from the retropie setup.

Sometimes it does not work and must be installed manually.

This is what I did, but you have to adapt it to your kernel version!

wget http://www.niksula.hut.fi/~mhiienka/Rpi/linux-headers-rpi/linux-headers-3.18.10-v7%2b_3.18.10-v7%2b-2_armhf.deb
wget http://www.niksula.hut.fi/~mhiienka/Rpi/gamecon-gpio-rpi-dkms_1.0_all.deb
wget http://www.niksula.hut.fi/~mhiienka/Rpi/db9-gpio-rpi-dkms_1.0_all.deb 
sudo dpkg -i linux-headers-3.18.10-v7+_3.18.10-v7+-2_armhf.deb
sudo dpkg -i gamecon-gpio-rpi-dkms_0.9_all.deb
sudo dpkg -i db9-gpio-rpi-dkms_1.0_all.deb 

To load it:
sudo modprobe db9-gpio-rpi map=1,1

To load it every boot put db9-gpio-rpi map=1,1 in /etc/modules