Alif depends on your backend requirements. As you may know Rails is all batteries included where as Sinatra is a micro framework. The GraphQL ruby gem supports and works well with both Sinatra (Sequel dataset) and Rails (Active Record).
May be you can try out yourself and see what works for you. Here are some examples for ruby: https://github.com/gauravtiwari/graphql-server-examples (a bit outdated though)