I think its a little over the top to suggest gsps improvement is mainly because of the steroids.
To me he is like a barry bonds or manny ramirez where they would have been great regardless of whether they took anything.
A lot of guys have taken stuff and very few if any are as athletic as gsp, have continued to develop in all areas as gsp and have his work ethic.
He is rich now and has the resources to seek out the most expensive drugs even when he was an unknown fighter he was still a great athlete with a high skill level and a very good fighter.
Of course the drugs would help particularly with recovery time and being able to train as hard as he does. But he seeks out the best trainers, coaches, and teammates, he is incredibly disciplined and as i mentioned already a great athlete. Even with the juice you still have to be able to develop the skills and techniques and gsp has done that becoming a better takedown wrestler than national champs, and also becoming a black belt in bjj while also being a dangerous striker.
A guy like Alves seems to have benefitted much more from roids. I havent seen his skills improve that dramatically. He still uses only stand, and his striking while dangerous is not techincially supreme particularly his defense. The main difference is that he has grown significantly which makes his takedown defense anda bility to get off his back that much better. He also has more power with his strikes.