On average, yes it makes a difference.
I'd agree with Kai that the GT teacher being gifted themselves can be a key ingredient.

Few teachers have any idea of how to effectively differentiate to a regular class room group, let alone to a class of GT students. To be honest, few teachers want to go to that much work which is why GT students are often just handed busy work rather than work that expands upon breadth and depth. At least with a certification they usually have some idea of the concept, though applying it is a different story ask Kai seems to be signaling.

As for the 12 credits, I'd have to say that it depends on where those credits came from. As with any other certification, degree, or education, some sources are much more effective than others.