I have used HTTP Request sampler for GraphQL testing with setting
request body with escaped json strings like this:

{"operationName":null,"variables":{},"query":"{\n fineSomethings(text:
\"\", offset: 0, limit: 200) {\n offset\n limit\n count\n total\n
items {\n ... }\n }\n }\n }\n}\n"}

It is a bit harder to read and update than some graphql tools such as
graphql playground [1] which escapes graphql and variable json to an
escaped body under the hood.

It would be nice if we have a GraphQL Sampler with a
similar UI - separate graphql query or mutation input and variables
(json) input.

GraphQL request over HTTP supports both GET and POST methods in the specification and some implementations. [1][2]
It seem that only POST method is used in real practice though.

* Related e-mail thread: [3]

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[1] https://graphql.org/learn/serving-over-http/
[2] https://www.apollographql.com/docs/apollo-server/v1/requests/
[3] https://lists.apache.org/thread.html/r43d8f18fc01bf2aca242fd757814c6440b465ffb4141e4d21fa04b34%40%3Cdev.jmeter.apache.org%3E