Gulp is accessible in VS Code using the Integrated Terminal, that you can access from View | Integrated Terminal. By typing gulp without any extra parameters, Gulp will run a default task. You can also run gulp from a Command Prompt or PowerShell window.
Tasks are described in a separate configuration file within the project folder called gulpfile.js. It is rather small in the SPFx default project, as a lot of work is done through SPFx tooling (sp-build-web, for example), but it is more than sufficient for your typical needs as a developer:
Line 6 is key here; it runs initialization for gulp as part of the build process. To understand what's going on here, we need to locate @microsoft/sp-build-web, which is one of the packages we installed earlier. By initializing the package, we automatically import and configure the necessary build tasks for a web browser-based (SharePoint Workbench in this case) build target.
To see what we can do with Gulp...