I'm pretty new to gql, but if I run a mutation that requires auth, is there any way to get the current authed user into the mutation - i.e. to say that "this thing belongs to the authed user"?

All Graphcool functions get information about the currently authenticated user in the context. For 'normal' mutations, this is not possible out of the box, but something that can be done using an API Gateway: https://github.com/kbrandwijk/graphcool-gateway-examples/tree/master/linking-node-to-current-user

Thanks.