A Gulp.js developer uses Gulp.js, a task runner in JavaScript, to automate repetitive tasks in the development process. This includes minifying code, compiling source files, running unit tests, optimizing assets like images, and reloading the browser automatically whenever a file is saved. The developer creates and manages tasks in a Gulpfile, enabling efficient workflow control. By automating these tasks, the developer enhances productivity, reduces errors, and ensures consistency. The Gulp.js developer may also be responsible for debugging Gulp tasks, integrating Gulp into build processes, and keeping Gulp plugins up-to-date.