Hello @allenh1:
Thanks for your efforts of maintaining Gazebo in Gentoo. According to the information in the Gazebo website, currently gazebo7 and gazebo8 are supported.
I think that one of the valid points you can consider to support one or more versions of Gazebo ebuilds in the ros-overlay is how many ROS versions you want to support in that overlay: gazebo7 is the version supported in ROS Kinetic and gazebo8 is the one used in ROS Lunar. This can help Gentoo users to have the same simulator experience than the users on Ubuntu/Debian.