Gulp is a tool that lets you decide what the build process should look like.
Webpack is a tool where you define entry points, split points etc., and what you finally need. Based on that webpack decides how to go about the build.
Most of what Gulp does, can be replaced with NPM scripts.
It's also very easy to find build bottlenecks with gulp which is very handy if you have a large project with a slow build. It allows you to scale out efficiently.
Yes, production (“non-watch”) builds can take a while. But who cares? You shouldn’t be running a production build after every code change. That’s why the hot reload server is so useful; it only recompiles the code you changed.