Login or register (free and only takes a few minutes) to participate in this question.
You will also have access to many other tools and opportunities designed for those who have language-related jobs (or are passionate about them). Participation is free and the site has a strict confidentiality policy.
Explanation: Avertir in english is to warn. I think you should say either "I will warn the gentleman..." or "I will inform the gentleman...
What is the context? Do you have to warn the gentleman about a danger or something? If you just want to tell him something, I would use inform. But still, both are good.