I would not recommend using the ROS distros as identifiers for the gazebo packages: they are both independent projects with their own names and versions, it is probably more consistent to keep the naming/versioning scheme of both. I, as a gazebo user/dev, would be surprised to see gazebo versions named with ROS release names. It should be easier to create gazebo7and gazebo8 and make ebuilds to depend on the correct version or create a virtual named gazebo (it should make maintenance more easy and I have some valid use cases where you can use gazebo7 or gazebo8 with the same version of ROS)