There are two standards for PTO rpm; 1000 RPM and 540 RPM.
The first PTO standard was adopted by ASAE (the American Society of Agricultural Engineers) in April 1927. The PTO rotational speed was specified as 536 ± 10 rpm; the direction was clockwise. The speed was later changed to 540 rpm.
Newer types, supporting higher power applications, operate at 1000 RPM.
So where does that leave the 24G with it's 1800 RPM PTO speed? Gravely had a speed reducer that it sold with a snow blower that would reduce the front PTO speed by a 2:1 ratio. That is why it is recommended that the engine speed be dropped to 1800 RPM when operating the tractor without the speed reducer.
Now that I have the data regarding the PTO RPMs, that mower deck that I took a video of was spinning with a blade tip speed of nearly 30,000 SFPM with the engine at 3100 RPM. The legal limit is 20,000 SFPM. One can calculate the proper engine RPM for a given mower deck when powered by the 24G without a speed reducer and still stay within the legal limits. The 50" deck spins at about 15,000 SFPM with the PTO shaft spinning at 900 RPM. Each mower deck is different. If I recall correctly and according to my sources, the 30" deck normally spins at about 17,000 SFPM.
Personally I like having the capability of more speed. When conditions warrant it, I have that flexibility.