bigfred 

To the best of my knowledge and I may be incorrect about this:

The GP 4 Season utilizes the same casing material and rubber compound as the GP4000S, but, with the duraskin and two layers of puncture protection.

I've been using 25/28mm GP 4 Seansons for several months. Excellent puncture protection, I haven't suffered a single puncture related flat. However, I have damaged two casings. One resulting in a flat. In one instance I found a lump in the contact patch of my rear tire. Upon dismounting I found that I had probably ridden over a sharp edge that had caused the cutting/breaking of several casing cords internally, while leaving the tread surface undamaged. In the second instance I picked up some piece of road debris which lodged between my tire/fork & front brake. Before I could stop it popped free, but, not without causing a small cut through the duraskin protection and the casing, just outside the edge of the tread. This resulted in the tube protruding ever so slightly and flating in short order.

In general I have been happy with the GP4 Seasons. These two instances have happened just recently. I feel they are more durable than the Michelin Pro Optimums I was using previously, but, give up a little road feel compared to the Optimums.

Mounted to 19mm DT Swiss 585 rims and inflated to 8bar/116psi my 25mm front and 28mm rear measure 25.5mm and 27.5mm respectively.

I've not ridden the Hardshells, but, have mounted up a few for friends. The casing of the GP4Season is less stiff and the tire lighter. Don't expect the same sort of mileage from that GP4seasaon that you probably get from the Gatorskin Hardshells.