How can I recover an old account that I only remember the gamer tag of

Yes they were live accounts that's why they had gamer tags. A few years ago you can set up a gamer tag on the Xbox without the link to a Microsoft account so how do you recover these old gamer tags without a Microsoft account?

How can I get my old gamer tag....I can't remember the email

If you remember the login information for your old account, you can recover it to the new Xbox. Instructions on recovering accounts can be found here -

support.xbox.com/.../gamer-profile

From what i can remember... if a gamertag is changed by the owner, the OLD gamer tag will be recycled after 6 months. Silver accounts do not delete.

The xbox feature is part of your Microsoft account. Not all accounts have a gamer tag, but all xbox accounts are part of the Microsoft account. The gamer tag which is not a required feature of the Microsoft account is so that gamers can represent themselves
in a public manner to other gamers without revealing their actual account (which is the account email).

The gamer tag is not part of Microsoft security in any form, it may help you recover a lost account but just having a gamer tag will not give you access to or is part of the security for a Microsoft account. I did Xbox support for three years, I helped
recover many accounts while customer support was able to do that. I have never heard of a secondary gamer tag before. The only way to recover the account now is through the compromised account link listed above.