It's certainly up to you and your application, and some libraries will automatically convert to and from camelCase, but given the choice, the GraphQL Rules website highly recommended that you just commit to using camelCase for all GraphQL fields and arguments, then convert those to camel_case fields in Ruby where necessary.
From the GraphQL Rules website website:
Rules and recommendations mentioned here were the results of 3 years' experience of using GraphQL both on the frontend and backend sides. We also include the recommendations and experience of Caleb Meredith (PostGraphQL author, Facebook ex-employee) and Shopify engineers.
Naming rules
1.1. Use camelCase for GraphQL-fields and arguments.
1.2. Use UpperCamelCase for GraphQL-types.
1.3. Use CAPITALIZED_WITH_UNDERSCORES to name ENUM-types.
I wish a bit that I had followed these for some past projects, but hey, they are still running just fine though :), but there has been churn where occassonally we were faced with some inconsistencies and chose to convert some fields.