Starting today at 2PM EST, Microsoft will be making available almost one million Gamertags from unused accounts to active Xbox Live Gold subscribers. The tags will be released over a 24-hour period in order to give subscribers in different time zones a fair chance of getting one of these released tags. These tags are being released from accounts that never moved to Xbox 360 and thus have not been used for quite a long time. Microsoft is not providing details on any of the names being released, they state that many of them are in the following categories.
Source: https://www.bleepingcomputer.com/news/microsoft/almost-1-million-xbox-live-gamertags-being-made-available-today/