It is arbitrary queries though? I can send any query that matches your schema and your graphql engine is probably going to produce some gnarly stuff to satisfy those queries.
There are of course products that do this automatically, but it's not really that simple. There's a reason things like Hasura are individual products.
The GraphQL structure can be totally independent from your DB if need be, and (GraphQL) queries on those types via API can resolve however you need and are defined by you. It's not a SQL generator.
On the other hand, unless you've put some serious work into hardening, I can craft a GraphQL query to your system that will produce way more data (and way more load) than you would prefer.
A mature GraphQL web API (exposed to adversaries) ends up whitelisting queries. At which point it's no better than REST. Might as well just use REST.
(I'm not aware of any, but if there are actually gems or libraries that do expose your DB to GraphQL this way, that's not really a GraphQL issue)