There are many Gatsby plugins that integrate with hosted cloud services, also known as SaaS (Software as a Service). You can search for Gatsby plugins as well as Gatsby starters that integrate with hosted services to source content and functionality. Some examples include:

These plugins will pull data from a hosted service and make it available to your Gatsby site via GraphQL.

More examples are described on the Gatsby blog:

You can also write your own source plugin to integrate with a service that isn’t already represented.