Which username? Cause GTR is registered to some completely different person. I'm assuming your own nick is already registered? Otherwise nickserv will ask you to either provide a password or choose another name. Like, it actually sends you a private message:
Nickserv
"This nickname is registered and protected. If it is your nickname, type /msg NickServ IDENTIFY password. Otherwise, please choose a different nickname."
If you don't get that message, it means you aren't actually registered, and if oyu want to register your nick, you type
/msg nickserv register password email
If you want more information on how to use the identify or the register command, type
/msg nickserv help identify
or
/msg nickserv help register
and it will spew out instructions about everything it knows, including how to AUTH your registered nick fro the email you provide, and how to set it so your email address is hidden.