Link

import { Link } from "gatsby"

<Link to="/">Home</Link>

Longer Version

import { Link } from "gatsby"

export default () => (
  <div style={{ color: `purple` }}>
    <Link to="/contact/">Contact</Link> 
    <Header headerText="Hello Gatsby!" />
    <p>What a world.</p>
    <img src="https://source.unsplash.com/random/400x200" alt="" />
  </div>
)