I was stuck with the same issue some minutes ago. After many unsuccessful queries using search engines, I checked the log to try to understand why those input methods weren't working.
In my case, installing the following 2 ports solved. I hope that it can help you out.
cd /usr/ports/x11-drivers/xf86-input-keyboard && sudo make install clean
cd /usr/ports/x11-drivers/xf86-input-mouse && sudo make install clean
After installing those, I rebooted my FreeBSD box and was able to get through the gdm login window.
[ PS: same answer as given in http://unix.stackexchange.com/a/72402/37151, since it is a similar issue from my point of view. ]