json response is 'plain/text' type. Anyway to change type to 'application/json'?

If you don't mind doing assertions through code, you could use JsonSlurper to take the response string and make it into an object like:

import groovy.json.JsonSlurper

String responseStr = "" // wherever you get the response from
def response = new JsonSlurper().parseText(responseStr)

// Perform assertions here (just making this stuff up)
assert response.employees[0].id == 12345

response.employees.each {
   assert it.salary > 0
}

Provided you're not trying to assert on some crazy complex stuff, this could work.