You need to have a recent nvidia-driver with LINUX compatibality support or an AMD graphics card that is supported by FreeBSD. See FreeBSD Wiki for further information.
nvidia-driver or xf86-video-ati
pkg install deb2targz rpm2cpio doas
Create /usr/local/etc/doas.conf :
echo 'permit nopass keepenv yourusername >> /usr/local/etc/doas.conf'
Convert it in to tar.gz:
Extract steam.tar.gz as root:
tar -zxvf steam.tar.gz -C /
Setting up SteamOnFreeBSD:
git clone https://github.com/SteamOnFreeBSD/SteamOnFreeBSD.git
For amd64 first run install.sh then run install_x86_64.sh, also in script steamrun change variable ubuntu on ubuntu_x86_64.