As I understand it, they are essentially very different technologies. GraphQL focuses on providing a DSL for requesting subsets of data from a web endpoint that "talks" JSON. gRPC-Web is just a wrapper around gRPC so that you can expose your RPC endpoints in a front-end friendly manner.
In other words: GraphQL is akin to SQL, while gRPC-Web would be closer to a serialization/exchange format like XML/SOAP.
In other words: GraphQL is akin to SQL, while gRPC-Web would be closer to a serialization/exchange format like XML/SOAP.