Please note that the ID/PW for your linked platform is not held by the devs and cannot be accessed directly. 

If you have forgotten the ID/PW of your linked account, please contact the Customer Support of each platform. 

■ Refer to Customer Support pages below
 
 [Google]  https://support.google.com/accounts/answer/7682439
 [Apple] https://appleid.apple.com/kr/#!&page=faq