Just because you are unable to find the gamertag does not mean it is available. This is from the FAQ
[quote]
I can't change my Gamertag (GT).
Most of the time if you are seeing a message saying the GT you want is unavailable, it’s true. One thing to remember is that spaces and capitalization don’t make your GT unique. For example, “Major Nelson”, “MajorNelson”, and “MaJoRnElSoN” are all the same
GT.
GTs are also tied to services like Zune or Games for Windows LIVE. If a GT doesn't exist on Xbox, it might be in use on one of those services. If you try sending the GT you want a friend request and it says it’s not a valid GT that just means it isn’t
used on Xbox LIVE, but it could be used for Zune or GFWL.
[/quote]http://forums.xbox.com/xbox_forums/xbox_support/f/12/t/153151.aspx#gt