Hey Johnny!
Webpack is being used in conjunction with gulp. They are not mutually exclusive. Gulp is the task runner, webpack is the module loader. All webpack is doing here is allowing you to organize your javascript easier. Just like you would have a main.scss and import _variables.scss and _layout.scss and such: webpack makes is such that you can require javascript files from your main.js.
Node is not running on the server. It’s running on your development machine to compile your assets locally. After compilation they are uploaded to a target server on deploy.