It is my understanding that PhysX is more comparable to a physics engine, like Bullet Physics or Open Dynamics Engine, both of which Gazebo integrates. Perhaps they could now integrate PhysX as well...
EDIT: When I say "integrates", I actually mean "interfaces with"; you can run Gazebo simulations using either physics engine (although switching between the two is not quite as simple as that).
EDIT: When I say "integrates", I actually mean "interfaces with"; you can run Gazebo simulations using either physics engine (although switching between the two is not quite as simple as that).