Road Bike Tires

Flo recommends that you run 23mm tires with the wheels. I know that other sizes will work, but don't know specifically about 28mm. Shoot Flo an email - they'll definitely respond. Good luck.

Not compatible for optimal aerodynamics but if you want a nice ride, lower Crr then the 28C will work as long as they fit within your fork and chainstay spacing.  Nothing different than me running the GP4000s 23C on my old 17C Felt rims where they stuck out like a light bulb but rode real nice.  From bicyclerollingresistance.com for the GP4000 (assuming you are using these tires?):

"On a 17C rim and an air pressure of 100 psi, all versions of the GP4000S II are a bit wider than specified. With the 23C being 25 mm wide, 25C 27 mm wide and the 28C a whopping 31 mm wide"

IIRC, the Flo rims have an internal 19.5mm width so if you are going to use these tires the 25C & 28C tires will likely measure slightly wider than stated above, but you probably won't need to be running them at 100 psi either. 

FLO's aren't light but what goes up must come down!

Tire manufacturers frequently give compatibility charts like this one from Schwalbe

https://www.schwalbe.com/en/reifenmasse.html

Just look across the top for your rim width, and then scan down for the tire widths that are considered by the tire manufacturer compatible.  I didn't look for GP4000 compatible charts but they are probably available too.

Same advice as above for checking clearances.  If you go up in size, it is probably worth checking your frame after a ride to see if there is evidence of the tire rubbing.  The torque of pedaling and braking and the lateral load from rocking the bike as you pedal pulls the rear rim toward the seat stays.