test gist

file1.js

//source https://gridsome.org/docs/taxonomies
api.loadSource(store => {
  const posts = store.addContentType('Post')
  const tags = store.addContentType('Tag')

  // makes all ids in the `tags` field reference a `Tag`
  posts.addReference('tags', 'Tag')

  tags.addNode({
    id: '1',
    title: 'The author'
  })

  posts.addNode({
    id: '1',
    title: 'A post',
    tags: ['1']
  })
}

file2.js

//source: https://gridsome.org/docs/taxonomies
query Tag($id: String!, $page: Int) {
  tag(id: $id) {
    title
    belongsTo(page: $page) @paginate {
      totalCount
      pageInfo {
        totalPages
        currentPage
      }
      edges {
        node {
          ... on Post {
            id
            title
            path
          }
        }
      }
    }
  }
}

file3.js

//source: https://gridsome.org/docs/taxonomies
<template>
  <Layout>
    <h1>{{ $page.tag.title }}</h1>
    <ul>
      <li v-for="edge in $page.tag.belongsTo.edges" :key="edge.node.id">
        <g-link :to="edge.node.path">
          {{ edge.node.title }}
        </g-link>
      </li>
    </ul>
  </Layout>
</template>

<page-query>
query Tag($id: String!) {
  tag(id: $id) {
    title
    belongsTo {
      edges {
        node {
          ... on Post {
            id
            title
            path
          }
        }
      }
    }
  }
}
</page-query>