I can tell you exactly.
I had a Gulfstream for 10 years and moved up to my current Marlin.
The ride is predictably smoother in a Marlin due to the extra length, beam & weight, although it is not a "night and day" difference. The Gulfstream's beam-to-length ratio, (Beam: 9' 3"- Length 23') makes it a remarkably stable 23 footer, especially in beam seas adrift. It is a really big 23 foot boat and rides like one.
The biggest difference between the two is the Marlin's advantage in a head or quartering sea, and stability in a following sea underway. This is due to the size & weight of the Marlin, as I mentioned earlier.
They both will pound in quartering wind driven seas 2 feet and over and if it's a blustery day, you'll take spray with both of them. The Marlin will ride a bit softer and be a bit drier. Knowing how to properly trim & tab the boats make a huge difference.
If you downsize to a Gulfstream, you'll miss the Marlin's "big-boat ride." The stability, the "safe" feeling you get with a 30 Ft Grady.
What you'll gain is a really rockin' 23 foot Grady that doesn't ride as well, but can still be classified as a good riding boat under most reasonable conditions. It can go 10 rounds with most any other 23 footer and win. Plus, you'll get better fuel numbers, an easier boat to horse around a dock if it has twins, and all the other attributes associated with downsizing.
If I wanted a smaller boat than my Marlin, I'd seriously consider going back to a Gulfstream. One thing I would avoid is a Gulfstream without twins, especially if your used to a Marlin. A single engine Gulfstream is hard to dock in wind and/or current, it doesn't have the "lift" that twin props give and unless you have a 300 or 350, they're underpowered.
Got long-winded, as I usually do. Good Vibes sent your way and I hope it works out!