Up to 1 million gamertags being freed up tomorrow from 7pm UK time!
https://majornelson.com/2016/05/16/recycled-xbox-gamertags-releasing-may-18th/
Best way to see what might be free is to use the following link but put in the gamertag you'd like at the end instead of XXX (although it looks like XXX might become free too). If it's a real profile with a blank avatar and 0 gamerscore then there's a chance it will become available.
https://account.xbox.com/en-GB/Profile?gamerTag=XXX
Things like Pizza, Taco, and Burrito will likely become free which is kind of funny. Who knows what other OG names people will nab.
There's a couple I'd like to grab but not sure if I'll get them in time as loads will be trying I imagine. You need to have been a gold member for at least a year to get one - probably to avoid people just creating accounts to reserve loads of good gamertags.