Now if you dont remember your gmail, have you ever used your gmail to send an email to a friend? If so you may try to get ahold of one of them and ask them if they can let you know what your email address is. If not, the only option is to reset the device using the hard keys. Here are the steps to reset:
Hard Reset (for frozen screen) keys:
1). With the Optimus S turned off, take out the battery and reinsert it
2). Press and hold ‘Home’ and ‘Volume Down’ button until the device turns on
3). The screen will guide you to the rest of the steps to reset your Optimus S
Make sure you have already backed up all data since the hard reset will erase all data stored and the apps. This reset will then reboot (restart) your phone to a point of default factory setting.