The general assumption seems to be that the redhead is just a friend. Gibbs flirts a lot for a man who has a girlfriend/if you remember the blond he kissed in S2,ep.12- " Doppelgänger"/, if that is what the redhead is, but he does not seem the type to cheat or be a player.
We never find out who she is as a character she isn't given a name. She's just the mysterious redhead in the silver convertible who picks up Gibbs occasionally. lol. I do know in real life that it is Don Bellasarius'(sp?) wife.
She was never given a name, a face, or a real plot concerning who she is. She was probably a girlfriend or a lover of some sort, but she seemed to disappear after the first season. She was probably written to show off Gibb's mysterious nature, and his love of red heads. :)
This is as much a question as an answer. I think the red-head is Mark Harmon's wife Pam Dawber w/a red wig to fit the well known preference of Leroy Jethro Gibbs.The character is probably a friend with occasional benefits.:o)