I've used 23mm GP4000S on HED Ardennes+ wheels for about 3 years. The rims are 20.7mm inside, 25.5mm outside width. These tires work great on the rims, being an effective 26mm.

Measured tire widths and air pressure (I'm 170 pounds) :
23mm GP4000S: 26.5mm. 80-85 psi front, 95-100 psi rear. Fast and reasonably good on chip-seal (gravel over tar surface) Yes, the tires are only a mm wider than the rim. They don't look flattened or otherwise distorted, though
.
25mm GP4000S: 29mm. 65-70 psi front, 80-85 psi rear. A new tire recently measured 28mm. Very good on chip-seal. These tires are only 10-20 grams heavier than the 23. So I'm wondering why I kept using the 23 -- I thought the weight difference would be higher. The 23 does looks good on there -- "aero".

The 25mm would be really nice on a "long distance" / "all-day-ride" bike, lightweight and fast, but effective on rough pavement.

38mm Compass Barlow Pass smooth tread: 39mm. 38 psi front, 45 psi rear ! I'll try about 5 psi less on unpaved roads. Floats over chip-seal and tar strips. It's surprisingly efficient on smooth roads, too. (but the tires+tubes are quite heavy.)

~~~~

28mm?
I don't feel the need for 28mm. They would be about 31-33 mm actual width, I expect. I'm guessing about 45-50 psi front, and 60-65 psi rear. That would be plush. Very helpful if there's lots of bad road surface.

But, they might be a tight fit under caliper brakes, and on the frame.

~~~~

Tire pressures
This bike pressure calculator gives me reasonable numbers.
Use the second section, with combined bike+rider weight, and 45%-55% weight distribution.

~~~~
Tubeless ready
I recently had a 30 mph front pinch flat on my 23mm GP4000S from an unnoticed egg sized sharp edged rock. Watch those downhills! The tire beads stayed on the rim, since it's tubeless ready (I use tubes) and the beads snap in place with a loud pop when first inflated after mounting. (Still had a speed shimmy, but I controlled it.) No scrapes or dings on the rims even after the hard hit on the rock.