A gevent developer uses the gevent library in Python for asynchronous I/O functionality. This involves writing code that can handle multiple tasks at the same time, improving efficiency and performance. They work with greenlets, lightweight, cooperative routines that allow Python programs to multitask. They are responsible for managing network-related tasks, handling event-based programming, and troubleshooting concurrency issues. Often, they also work on web servers or applications that need to handle many concurrent connections. They ensure that the code they write is efficient, reusable, and easy to maintain while maximizing the capabilities of the gevent library.