Okay, there's nothing down below. That's all I wanted to know. I have plenty of boat(s) right now. I have no axes to grind, bones to pick, or threads to troll. I was simply looking at the website and wondered. Let's end this topic because someone seems to have his cranky pants on.
I think the only real difference is that the GP 26 doesn't have any cushions in it in these photos.
When I designed the interior of the Mumm 30, we put a stove, sink and cooler moldings in the boat. These turned out to be the most useless items in the boat - impossible to use effectively on a smaller high performance yacht.
I'm afraid the nice photo of the Dart interior with the kids and cushions is quite a different reality with a grown-up sitting down below in 20 knots.
I doubt you'll want the cushions/giant sponges onboard for that.
The newest GP 26s have built-in storage bins in the topsides section of the keel frame; you'll want to keep your wallet and cell phone up out of the bilge. You can also fully fit the boat with cushions for weekending, and I suspect the boat may actually be more comfy that the Dart due to it's size.
Below are photos of the pretty Dart interior vs. the reality of a sportboat drying out after a days sail (Mumm 30).