Welcome to the Postman Community!
Variables in GraphQL are distinct and are different from variables in Postman. They work hand in hand to complement each other. You will notice that the GraphQL client has a seperate input field on the right for inserting GraphQL variables.
You will need to author GraphQL variables in your schema as you would if you were working with GraphQL in your client side code. Then use the GraphQL Variables tab I mentioned to inject Postman variables into your gql query.
Take the above screenshot for example, the gql variables were first declared and used in the schema. Then Postman variables are passed into this schema on RHS.