I would also contact the other airlines (after you confirm that you are an Elite with Air Canada/Aeroplan by phone, or (as Taupo indicated) by fax, or by email - you can send an email to Aeroplan through the Aeroplan web site), for a variety of reasons. One is to confirm that the flight reservation is in their system - you can enter your FF#, request seat selection, enter contact phone info etc.

I would then ask the other airline if their computer has a notation that you are Star Alliance Gold, next to your FF#. If not, you may have to go back to AC or Aeroplan, to communicate with these other airlines about your new status. If the airline does have the Star Alliance Gold number, fine.

Then, in either scenario (either by phone, or preferably by email to the airline in question) indicate to the other airlines that you do not have a Elite/Star Alliance Gold card. Would the printout of the AC web site indicating Elite status (I am not sure if the web site actually specifies your Star Alliance Gold status) would suffice, along with the boarding pass, in the absence of your Elite/Star Alliance Gold card. If you get email confirmation that this is okay, it is easier to get the lounge agent to allow you entry.