@valem If you are simply talking about designing a simple hexacopter, it is not necessary to evaluate your design in gazebo.
Gazebo models are not used for evaluating a design of an airframe, but rather if you model the airframe dynamics into gazebo, we can prototype on the software faster. That means, when you model the airframe into gazebo, you need to throughly evaluate whether if the real model is similar to the sdf defined.
Multirotors in general have simple dynamics. This means you can be a little naive on envaulating your design and just try and fly it.
Or am I missing something here that you have a special reason you need to evaluate your design?