Need help retrieving old 360 gamertag.

You could try what Onxy suggested or you could also try searching
here.

Enforcement cannot do anything for you. They only are there for suspensions and bans. You will need to call back to phone support for any help with this as no one on this forum can help you. You would need to prove with them with out any doubts that
you are the true owner of the account. They should be able to see it.

Anyhow I wish you the best of luck getting it back.

Kiesey

Is there an actual place to look up gamertags on the xbox site?

You could try the search function at the top of this page but that would only give you tags that have been used on these forums.

There are so many services within the Microsoft family (external and internal) that it would be impossible for anyone outside of working for them to know if a tag was used or not.

After a certain amount of time of inactivity, gamertags are released. Looks like someone already has Killer D So unfortunately you wouldn't be able to get your original gamertag back, As the user said above the enforcement team wouldn't get involved in
something like this,