Take a look at the documentation on testing within Grails. It covers specifically how to set parameters for testing controllers. From the examples:
import grails.test.mixin.TestFor
import spock.lang.Specification
@TestFor(PersonController)
class PersonControllerSpec extends Specification {
void 'test list'() {
when:
params.sort = 'name'
params.max = 20
params.offset = 0
controller.list()
then:
// …
}
}