A Gmail API developer uses Google's Application Programming Interface to interact with and manipulate Gmail services. Their work involves accessing user's email threads, messages, labels, drafts and history. They can send emails, filter spam, and manage email settings on behalf of users. They also create applications that integrate with Gmail, enhancing its functionality and user experience. This requires proficiency in coding languages like Python, JavaScript or .NET. Additionally, they must ensure compliance with Google's API usage policies, maintain the security of the data accessed, and troubleshoot any issues that arise during development.