当然，我一贴出这个，就找到了我的答案。

import org.codehaus.groovy.control.CompilerConfiguration
import spock.lang.Specification

class MyTestSpec extends Specification {
    Integer getOne() { return 1 }
    Integer getTwo() { return 2 }

    void 'call script with local methods'() {
        given:
        CompilerConfiguration cc = new CompilerConfiguration()
        cc.setScriptBaseClass(DelegatingScript.name)
        GroovyShell sh = new GroovyShell(this.class.classLoader, new Binding(), cc)
        DelegatingScript script = (DelegatingScript) sh.parse("getOne() + getTwo()")
        script.setDelegate(this)

        when:
        def result = script.run()

        then:
        result == 3
    }
}