There are many commands available to help you build and test sites. Here are a few highlights to get started with.
$ gulp watchThis outputs an IP address you can use to locally test and another that can be used on devices connected to your network.
$ gulpBuild and optimize the current project, ready for deployment. This includes linting as well as twig compiling, image, script, stylesheet and HTML optimization and minification. It is made to create landing pages so it compiles Twig into HTML.
$ gulp pagespeedRuns the deployed (public) version of your site against the PageSpeed Insights API to help you stay on top of where you can improve.