Posted this on Gazebo Answers and realise it's a bug after reading the issues.
In the same terminal, I set up my path and load a model within that location.
I'm using a model which was able to load on Gazebo when I didn't set up the path. The .sdf (edited the mesh path) is from here.
The terminal gives me the following fail and doesn't launch Gazebo.
gzclient: /usr/include/boost/smart_ptr/shared_ptr.hpp:734: typename boost::detail::sp_member_access<T>::type boost::shared_ptr<T>::operator->() const [with T = gazebo::rendering::Camera; typename boost::detail::sp_member_access<T>::type = gazebo::rendering::Camera*]: Assertion `px != 0' failed. escalating to SIGKILL on server
I also recieved an Ubuntu crash report after this.