Having relocated to Turin, Italy, a country where I can speak the local language, I decided to enrol in what are called "acrobatic gymnastics" (for beginners) classes, which seem to be essentially adult gymnastics lessons. I was pleased to see how well the GB courses prepared me for it: protraction, PPT, proper shape in handstand progressions, and the like.
I have been following GB for about a year and a half, and I am definitely still a beginner, somewhere between PE2 and PE3 at most elements, but it is nice to see some results 
I'll go as far as to say that, as long as one puts effort and discipline in the training, and submits form checks often enough, the preparation received via GB in terms of conditioning, strength and mobility training is not at all inferior to that received attending lessons. In a group lesson, the attention an individual receives is hardly more than what you can get via a form check. A high level personal trainer would be a different thing of course, but also a very different price.
Of course, the social aspect of a lesson is a big plus, the instructor may notice defects which are not evident from the angle you filmed your form check from, and you don't have the hassle of filming yourself. Plus, at the classes we practice skill elements which are deliberately and understandably not part of the GB curriculum, but which for me are some of the most fun. On the other hand, GB is cheaper on the long run, and has flexible hours. The level aimed for as well as the attention for details in GB are also probably higher than those of the classes I am following, and I think I am at a good gym.
I will surely try to keep doing both!