How to gently inform the interviewer that the interview was unsuccessful?

It doesn't matter if you don't notice, but in order to show respect for the interviewer and show the atmosphere of the company, you can say:

After careful consideration, we have made a decision that your qualifications do not meet our special requirements. Therefore, we mentioned earlier.

Pleasedthattourdecisionis no way questioning your ability to succeed in other applications. Our special requirements may not affect other organizations.

Thank you for your time and opportunity to consider your application. We hope that every success will bring you surprises.

After careful consideration, we feel that your qualifications do not meet our special requirements, and we can't provide you with the position you applied for.

Our special requirements may not exist in other companies or positions, so please understand that our decision is not to doubt your ability to succeed.

Thank you again for taking the time to consider our company. I hope you can pay attention to other positions in our company and wish you an ideal job.