Requesting ownership of a gateway linked to an unaccessible account

In short, I need to associate a Multitech gateway to The Things Stack Sandbox, but I don’t have access to the previously used credentials. A former colleague was responsible for carrying out these tests and it is no longer possible to contact him.
Is it possible to remove a gateway from the network without having the credentials of the person who initially configured it?

No and there are many many many many posts on this forum about this matter & how to resolve it.

Even so, I appreciate your response

Reach out to TTI and raise your question there. There is a fair chance they will help you.