build.gradle

apply plugin: 'tomcat'

buildscript {
    repositories {
        add(new org.apache.ivy.plugins.resolver.URLResolver()) {
            name = 'GitHub'
            addArtifactPattern 'http://cloud.github.com/downloads/[organisation]/[module]/[module]-[revision].[ext]'
        }
    }

    dependencies {
        classpath 'bmuschko:gradle-tomcat-plugin:0.8.1'
    }
}

repositories {
    mavenCentral()
}

dependencies {
    def tomcatVersion = '6.0.29'
    tomcat "org.apache.tomcat:catalina:${tomcatVersion}",
           "org.apache.tomcat:coyote:${tomcatVersion}",
           "org.apache.tomcat:jasper:${tomcatVersion}"
}

HelloServlet.java

package test;

import java.io.*;
import javax.servlet.http.*;
import javax.servlet.*;

public class HelloServlet extends HttpServlet {
  public void doGet (HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {
    PrintWriter out = res.getWriter();

    out.println("Hello, world!");
    out.close();
  }
}

web.xml

<web-app xmlns="http://java.sun.com/xml/ns/j2ee" version="2.4"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http:/java.sun.com/dtd/web-app_2_3.dtd">
  <servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>test.HelloServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
</web-app>