What is type "StringQueryOperatorInput"? How can I get rid of this annoying graphql error?

Actually literally right after I posted this question I found the solution. I needed to set up my query like so:

export const query = graphql`
  query($clientId: String!) {
    clients(id: { eq: $clientId }) {
      firstName
      lastName
      email
    }
  }
`;

I assume that leaving out the {eq: $clientId} throws that StringQuery error on the GraphQL side. I still do not know what a StringQueryOperatorInput is, however, I have successfully generated the pages with the data from firebase.

StringQueryOperatorInput is the type of the id argument of the clients field. GraphQL includes scalar types like String, ID or Int as well as types that describe more complex data structures like arrays or objects. In this case, StringQueryOperatorInput is an input object type -- it describes objects that can be used as inputs like argument values or variables.

When filtering fields, Gatsby uses an input object like this to enable using a variety of comparison operators to filter the exposed data -- in addition to eq (equals), you can use other operators like ne, regex, in, gt, etc. You can see the full list here. Because not all inputs apply to all scalars (regex makes sense for a String field but lte does not), there's a different input type for each scalar (IntQueryOperatorInput, BooleanQueryOperatorInput, etc.)

Gatsby exposes a GraphiQL endpoint in development. Writing queries using GraphiQL allows you to utilize autocomplete and syntax highlighting so that you can avoid unexpected syntax errors like this. You can also use the "Docs" button to search and browse the entire schema.