Hey @Doug_Osborne ,
the dirty way is in the gatsby-node.js
There might be a way better solution which might be the way @LuisAlaguna intended it to be.
It is a mixture of gatsbys createSchemaCustomization and onCreateNode with createRemoteFileNode.
As this might be a common case, I will write a small article about it as soon as possible (it is a little bit more complex than the dirty solution, but gives you the right types for your GraphQL Queries).
Will keep you updated.