Comment by ericyd
The author states that in their experience, most downstream services are REST, so adding a GQL aggregation layer on top isn't very helpful. It seems possible they would have a different opinion if they were working with multiple services that all implemented GQL schemas.
In that (common) case, the advantage is the frontend/app developers don’t need to know what a hot mess of inconsistent legacy REST endpoints the backend is made of, only the GQL layer does. Which also gives you some breathing room to start fixing said mess.