The tournament organizer will send a message with the code within 10 to 15 minutes from the tournament's start time to all the (accepted) and (checked-in) participants. This message will appear on your messages list, so try to be patient and contact the tournament organizer directly if you did not receive the code within 15 minutes.
GBarena is NOT responsible for sending room codes to the players at all, so please contact the tournament organizer if the code message is late.