Gridsome provides many source plugins that help you create collections from your source. But if you want to create collections from your API or a third-party API, you have to create collections manually.

In gridsome.server.js file, you call loadSource method of api parameter. You pass a callback function which you will fetch the API and create collections from data.

Your callback has an actions parameter, it has addCollection method. You will use this method to create your collections

const axios = require("axios")

module.exports = function(api) {
  api.loadSource(actions) => {
    const { data } = await axios.get('https://example.com/api/v1/posts')
    const postCollection = actions.addCollection("Post")

    for(const post of data) {
      postCollection.addNode({
        id: post.id,
        title: post.title
      })
    }
  }
};

After you start the server, it will create Post collection. To get the collection, you call getCollection method with the collection's name

actions.getCollection("Post")