(Apollo Federation) How to route gateway's own schema

Question

I am configuring a federation using apollo-gateway.

Many services can be provided through the gateway, but functions such as authentication would like to be provided by the gateway itself.

I am using supergraphsdl and implemented subgraph without using serviceList.

How can I express the graph provided by the gateway itself?

// index.ts
const apollo_gateway = new ApolloGateway({
    supergraphSdl,
    buildService: ({ name, url }) => {
        return new RemoteGraphQLDataSource(name, {
            url,
        });
    },
});

const apollo_server = new ApolloServer({
    gateway: apollo_gateway,
});

Since I'm using rover to create subgraphs as supergraphs, I don't know how to include the graphql of the gateway itself in the supergraph.

Can this be done?