Out of the box, I think this would be contrary to the goals of Apollo Federation given that an implementing service is intended to manage part of a graph, not an entire graph. You could try wrapping gateway B in an implementing service in gateway A, but you would need to redefine the parts of the schema you want to expose in wrapper service's type defs and resolvers. I've done this with portions of the GitHub GraphQL API inside of an implementing service before.
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Out of the box, I think this would be contrary to the goals of Apollo Federation given that an implementing service is intended to manage part of a graph, not an entire graph. You could try wrapping gateway B in an implementing service in gateway A, but you would need to redefine the parts of the schema you want to expose in wrapper service's type defs and resolvers. I've done this with portions of the GitHub GraphQL API inside of an implementing service before.