Just because you let your "Gold" membership lapse, that shouldn't mean they will release your gamertag. Log into XBL as a "Silver" ("Free") member once in a while and that would easily signal to them that you are still active on your XB360. If you don't have an internet connection for your XB360? Try logging in to xboxlive.com once in a while with your account that is connected to your gamertag. Either of those actions would easily signal to MS that you are still using the gamertag.
In regards to "all of the good names without numbers/symbols are taken"... WTF? You have 15 characters that can use any combination of 26 letters and a space. That's over 2.954 sextillion combinations... or 2,954 billion billion. Even if every one of the 30+ million XB360 owners had an active account on XBL, that's almost 100 trillion combinations for each owner. If a person is so bereft of imagination that they can't come up with a unique AND legible gamertag without resorting to 1337-speak... that's just sad.
And yes, the irony of these statements from a disposable user account made with a random combination of letters and numbers is NOT lost on this poster.