Another benefit of GM'ing style exercises is that it calls into play the important ham's. You will get a muscle involvement and stretch from the whole chain of rear leg muscles; ankles, calves, hams, hip/butt. And also adjust the back to having a weight on the shoulders. Hyper X's will not do that. Though you could, at some point, throw in hyper X's.
GM'ing are an extension exercise, though some lifters use them as a successful power movement. All depends on the foot spacing and amount of knee bend. Can also do GM'ing while sitting on a bench crossways. A more advance form is doing them while kneeling.
May also consider working the very important abs hard while retraining the lower back. They help with a more stable function of the lower back. And can help a recovering back injury rather quickly. Good Luck.
