Hi camac1,
Things have changed significantly with the NBN in the last few years since this thread was started. If you are using an NBN service you don't need a username and password saved in the modem to get an internet connection.
In it's generic set up, you do not need an username and password to access the TG797n V3's Graphic user interface either. Pointing your web browser to the modems location on the network - 10.0.0.138 should take to directly to the home page (below)
If you have modified it and added login details, which you can't remember, the best way to fix this is to reset the modem to it's factory default settings, so that you can access the modems GUI.
Reset the T-Gateway to factory default settings. To do this get a paperclip, or something similar, and press and hold it into the reset hole at the back of the T-Gateway, keep in pressed in for 15 seconds. Give it 2-5 minutes to reconnect to the internet.
Hope this helps.
Ben