I've ridden the following in 28mm:

GP 4000 S2 - They run big. The 28mm size measures 31mm on my rims. Great tires, though. But probably too wide for your application.

GP 5000 - These measure true to size in the 28mm size, mounted on the same rims on which the 4000s ran big. These are an upgrade in several ways from the GP4000sII. I have 28mm GP5000s on my road bike.

GP 4 Season - I've owned these in 28mm and 32mm. They seem to run true to size. And I find them fairly comfortable. Plus, they have good all-weather traction, and are resistant enough to flats that I've only had one flat in 1500 miles with them. I have 32mm GP 4 Season tires on my commuter/hybrid.

The 4-season and 5000 would be my vote for comfortable. Also remember in a 28mm size, you can run the tire pressure lower than you would run 25mm tires, and that alone will get you a more plushy feel.

Going from Gatorskins to any of the tires mentioned above will feel less harsh. Going from 25 to 28mm will feel less harsh. Going from 105 PSI to 90 will feel less harsh.