At the time of this writing, the registered user base for GuildPortal is approaching half a million users.  Because of that, more common fantasy-style names are already taken by active users.  Even though your name is unique in your guild, GP accounts are global to the service (for example, with a GP account, you can post in your allied guild's forums as well as your own, or even in the forum of a guild that's on another game that you play).

One suggestion would be to try the name again, but with a small variation like adding the abbreviated name of the game you play to the end of the name.  For example, if the login name you want is AxeMaster, then you could append the game to the end and try AxeMasterEQ.

That said, we do have an automated process that periodically changes the names of inactive accounts (accounts that have not logged in for a year or more) by adding a _ to the end of the name, thereby freeing up the name for re-use.