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