Congratulations on the release of Jazzy Jalisco!
The Gazebo team is happy to announce that Gazebo Harmonic 
is available for ROS Jazzy! Get started with:
sudo apt install ros-jazzy-desktop-full
See Gazebo Harmonic in action at the community meeting held right before the release and read the release post to learn about all the new features available.
ROS and Gazebo-Classic
As announced earlier, Gazebo-classic (aka Gazebo11, see Terminology) is no longer being actively developed and will not be available on ROS Jazzy (REP 2000). Instead, we recommend users to migrate to modern Gazebo (formerly known as Ignition). Please see the migration guide to get started.
Updates for Jazzy
For the ROS Jazzy release, the Gazebo team has been working in collaboration with the ROS team to improve the integration of Gazebo and ROS. We would like to announce the availability of Gazebo Vendor packages and give you a sneak-peak of upcoming features.
Gazebo Vendor Packages
Gazebo libraries are now available directly from packages.ros.org via vendor packages. The packages are built in the ROS buildfarm and as part of their build process, fetch the sources of the underlying Gazebo library and build it. In addition, the vendor packages provide CMake shims that make it possible to use CMake targets without version numbers. See the documentation for full details.
A special thanks to @mjcarroll, @clalancette, @cottsay, and @nuclearsandwich for their help and guidance in creating these vendor packages.
Improved Launch Files (Coming soon)
ROS launch files used to start Gazebo, spawn models in simulation, and start the ros_gz bridge are now much simpler and more idiomatic. New XML and YAML elements are available to simplify the creation of launch files. As an example, here’s how you can start gzserver and the bridge from an XML file:
<launch>
<gz_server world_sdf_file="$(find-pkg-share my_package)/worlds/my_world.sdf" />
<ros_gz_bridge config_file="$(find-pkg-share my_package)/config/bridge_config.yaml" />
</launch>
See issue to keep track of progress.
Improved ros_gz_bridge performance (Coming soon)
A new parameter use_composition is also available in all the new launch files to leverage the ability to launch composable nodes. This feature allows us to run Gazebo, the ros_gz_bridge, and other potential ROS composable nodes within the same process. This improves performance by avoiding message serialization and network transport between Gazebo and ROS.
See issue to keep track of progress.
Gazebo Development Team