Help:Single Sign On
GPWiki uses a single sign on policy to link your forum account to the wiki. This removes the need for you to remember two sets of account information, links forum and wiki usernames and helps keep spammers away.
To use the wiki, you simply have to sign up for an account on the forum. Once you are logged into the forum, you will be automatically logged in to the wiki too.
If this isn't working for you, post a message in the Help forum.