My $.02 -
Puffers thrive in extremely complex settings, whether they are aggressive with tankmates or not.
I prefer my aquariums pretty full, and my most recent setup (36g corner) is probably the most full and complex I've ever had;
There are 3-4 caves they can hide in, and tons of plants to explore. The back 1/3 of the tank, where all the caves lead to, is completely open and obstructed from view, so they can hide back there if they please.
Open swimming space is minimal, but none of my fish would take advantage of it if there was.
Try to think 3D; you want the tank to look nice to an observer, but most importantly, you want something that will make your fish feel safe and provide them with the opportunity to explore.
I think having plants at the top of the tank is as important as having them on the bottom for a sense of security.
Finally, with this setup, my Figure 8's are exhibiting more what I would consider "normal" behavior than ever before.
Pacing is almost non-existent; they are constantly occupied with puffer business.
Here is a setup in another tank I use (45g) that they seemed to enjoy (though not as much as the current one):
Looking at your picture, I think you could use a ton more decor, particularly solids. Free-standing plants like that do very little to make a fish feel secure IMO; your GSP has nowhere to hide, really.
There are some nice, natural looking fake caves that are relatively inexpensive, and real rocks are always nice, too, though a bit more expensive. And I'd recommend large floating plants to almost any puffer owner.