Gateway Wiki
How does it work?
you can check this github repository.
What are the username/password in ConnectionString?
It's all up to you, specifically up to your OAuth Endpoint.
What are the results of the Query?
Again, it's all up to you, specifically up to your DataProvider (WebApi Endpoint).
Could SqlClient use Integrated Security?
No. Windows Integrated Authentication is never an option for public service.
Is it secure?
TLS is mandatory on your OAuth Endpoint and WebApi Endpoint. The service use TLS 1.2 to communicate with sql client, and force encryption. If sql client does not support encryption, it will not be able to establish a connection to the serivce.