Looking at the rosdistro libgazebo7-dev rules (here) it appears that Fedora keys are only available up till version 25, which is the reason that you get this error.
This error means that rosdep is not aware of a binary release of Gazebo 7 for Fedora 26, and since you specify --ignore-src it will also not be able to satisfy that dependency with any package in your catkin workspace that provides it.
Searching a bit leads me to fedora packages - gazebo (linked from Install Gazebo on Linux distributions (non Ubuntu)) which shows that there are RPMs available of Gazebo 7, however I don't know of those pkgs are in an actual Fedora distribution, or are user provided pkgs (like in an Ubuntu PPA)).
If the former, then you could submit a PR to rosdistro which adds the 26 key to libgazebo7-dev pointing to the correct Fedora pkg.
If the latter: rosdep rules typically don't point to user contributed packages / repositories, so adding the key would not be acceptable in that case. The easiest is then probably to install the Gazebo 7 pkgs from fedora packages - gazebo and invoke rosdep with the --skip-keys="libgazebo7-dev [..]" argument, which will make it ignore that key. As you already have Gazebo 7 installed at that point, packages depending on it should still work.
Note: I didn't find (but also didn't search for) any binaries for gazebo_ros_pkgs for Gazebo 7 on Fedora 26, so you may have to build those from source in your Catkin workspace to make this all work together.
Note2: there appear to be multiple keys for Gazebo 7 in the rosdep database: libgazebo7-dev is only one of them (search for gazebo and gazebo7). The gazebo7 key does map to gazebo on Fedora (without any specific version spec) and looking at fedora packages - gazebo that could map to Gazebo 7 (or 8, I'm not sure - not a Fedora user).
