It sounds as if the Centurion Studio will not sell day passes to American Express Gold, Green or credit card holders:
Access to The Centurion Lounge is complimentary for global Platinum Card® and Centurion® Members. All other Card Members with a valid American Express® Credit or Charge Card are eligible to purchase access to the Lounge for $50 per day, subject to capacity.
Access to The Centurion Studio is exclusively complimentary for global Platinum Card® and Centurion® Members.
http://thecenturionlounge.com/info/f...ions-and-hours