I think the key to GPP is building it up, you have to start extremely light and then each time you can add a little more each time you do a GPP workout. You should keep them very short, about 30 mins.
When I was using the bear a lot for GPP I jumped into it too fast and got a lot of soreness. But then I backed off the weight, number of reps, and number of sets. I started with one set of 6 with light weight and then after one week added another set. My point being that you need to get your body used to doing extra workouts, if you do too much GPP you will hurt your lifts. But if you work up to a greater GPP it can help your lifts.