In Zabbix documentation 2.2 the following statement should be updated, while zabbix-java-gateway is available for download:

5.1.1 Downloading from Zabbix website

This option is not actually available yet, but in future you will be able to download Java gateway archive from Zabbix website.

More info (unfortunately in german): http://lab4.org/wiki/Zabbix_Java_JMX_Monitoring

Example for Ubuntu:

$ apt-cache search zabbix-java-gateway
zabbix-java-gateway - network monitoring solution - java-gateway

$ apt-cache show zabbix-java-gateway
Package: zabbix-java-gateway
Source: zabbix
Version: 1:2.2.6-1+precise
Architecture: amd64
Origin: Zabbix
Maintainer: Kodai Terashima <[email protected]>
Installed-Size: 827
Depends: java6-runtime-headless | java7-runtime-headless, adduser
Suggests: logrotate
Priority: optional
Section: net
Filename: pool/main/z/zabbix/zabbix-java-gateway_2.2.6-1+precise_amd64.deb
Size: 711128
SHA256: 34c848386125447cb94ac4dcaa0596e4e34009633133616d86673d978a6d849b
SHA1: 8efec8bac61bf5091504ecc28b385934d21d5ba0
MD5sum: 3c70dbcc4ca0f5e1b27e3f3476f52ce1
Description: network monitoring solution - java-gateway
Zabbix is a server/client network monitoring system with many features.
It can be used for:
.

• high level monitoring of IT services;
• centralized monitoring of your servers and applications;
• monitoring of SNMP-enabled devices;
• performance monitoring (process load, network activity, disk
  activity, memory usage, OS parameters etc.);
• data visualization.
  .
  This package provides the software needed to monitor a Java application
  using a Zabbix Java gateway. It collects information from JMX agent.