If you’re building dedicated APIs for just one client you’ve come full circle and building a monolith with a bunch of extra step. Why not just build a good old web app at that point?
Also with GQL you can separate by business domain as opposed to consumer for example your customer apps/website could use one gateway, and your back office app could use another.
Also, if you have an old monolith in something like rails, and want to build new functionality in something different whilst migrating parts of it over, it makes sense to have a service oriented architecture. A gateway is a layer that exposes all of the services in a unified and consistent manner.