Pure Strength is just an older version of Gym Styles (IMHO) and it splits the muscle groups up a little differently. For instance, GS does Back, Shoulders and Biceps one day while PS does CHEST, Shoulders and Triceps (which I think is a better way to split those muscle groups). I would substitute Gym Styles if you have that one. They're both good, but GS is a more thorough (and in my opinion) more difficult workout. I would split GS up the way PS is split, though if that is what the rotation calls for. You don't have to--it's just a thought. And a way to shake it up so that you're not doing Gym Styles exactly the same way two weeks in a row.