Since Gatsby is a Static Site Generator (SSG), you'll need to kick off a rebuild of your website by using a Webhook to tell your hosting provider changes have been made in your Agility CMS instance.
Webhooks allow you to create URLs that accept HTTP POST requests in order to trigger deployments and re-run the build process on certain events, such as Content Save, Content Publish, or even on Workflow Events.
Adding Your Webhook in Agility CMS
In your Agility CMS Manager, head to the Webhooks section under Settings and add a new Webhook. Paste in the Webhook URL your hosting provider generated for you. To make sure everything is working, you can Send a Test Payload and if successful, you should receive a successful Test Payload Response.