Might consider GM'ing while sitting on a bench/box/platform..etc. A bit more focus on lower and middle of the spinal erectors. Surprisingly the hams are also brought into play, though by looking at the exercise you might not think so, by attempting to keep the legs/hips/butt somewhat in place.. Can straddle a bench in a PR or back the weight off a rack, squat rack, etc.
A few others have preformed GM'ing with the butt flat on the floor and the legs spread out a bit wide. That one will get the ham stretch into play quite well. Bar is placed on the shoulder by training partners.
In training, a few athletes will stand and do one leg at a time Dl's/SLDL's. Gives a whole different slant on lower back training and ham development.. Usually a heavier DB is used. Reverse rear leg extensions are also well worth the time, either one leg or both at the same time.
Good Luck.
