the setting. GDMT is a ratified standard for ADSL modems. It allows users of the BT ADSL service to buy and install their own modems, as opposed to using the modem that is normally supplied by BT wholesale.
Unless a broadband modem is GDMT enabled it will not work on any UK broadband service.
Just relax and enjoy the Alcatel 330 - it's a great modem, and I use one myself.