So, I signed my dad up for what I thought was Gmail and GTalk, but
instead, he only got GTalk.

He has no difficulty logging into GTalk with a user name, but to log
into Google Accounts, he needs to use his Comcast email address. The
point was to get him outta Comcast!

Anyone got any ideas?

(BTW – I've tried signing up for GMail with his GTalk username, but it
says the GMail username is already taken)

Gtalk is now open to people without gmail. You have to send him a gmail invite from your gmail account, not gmail chat or gtalk. You can also sign up from the gmail homepage.