reboot the machine, look for: "Grub loading, please wait..." when you see this on the screen, hit "ESC". this will take you to a menu.
you would then want to select the menu item containing the words "recovery mode", after a few moments you will see another menu.
select "root Drop to root shell prompt"
hopefully you hadn't already created (and forgotten) a root password. if you had not previously created a root password you should see a "root@gOS:~#" prompt.
type, "cat /etc/passwd", you should see the username (created when you installed gOS) towards the bottom of this file. next, type "passwd username", follow the prompts and you should be able to get back to normal usage.
if you had at one point created a password for the root user, you may want to google for "recover root password"...
good luck.