Trying to Link GW to GW2.
When registering GW2 you have been asked to name a GW1-account. By linking them you will give your GW1-account a new email and a new password (if you choose to do so, which is good anyway).
If you failed to link the accounts you still can link them via the GW2-account page on this page. There is a point “linking accounts” (maybe it is called different, because I see the German version). There you should find the same options.
If you don’t know the necessary GW1-account information for this, you will have to ask support to help you in either to get those information or help you to link the accounts.
I hope this helps.