grpc is a bog-standard "shallow" RPC: client calls an endpoint, client gets a result, if client needs sub-results it calls more endpoints.

GraphQL queries have depth, when you call a toplevel endpoint which returns a non-trivial type you can select fields of the sub-resources to fetch, in fact you can also select just a slice of the top-level resource as well.

Let's say HN had a grpc API and you wanted the top comment of each article of the main page, unless it provided a very specific endpoint doing that you'd probably have to list the front page (and get a whole host of crap you don't care for e.g. the title, the destination URL, the score, the author, …), and for each article get its comments (again with all sorts of metadata you may not care for). In GraphQL, you'd select all but exactly what you need at once.