Even as kids we knew a propeller moves an aircraft through the air. What you may not be so familiar with is the variety of propellers available. Propellers used on gyroplanes can be made of wood, metal, or of composite construction. The propellers used can be fixed pitch, ground adjustable, or constant speed. You generally do not find two-position propellers on gyroplanes but that's not to say you won't either.
Some more interesting tidbits on propellers is their relationship to the engine. Remember in the certificated aircraft section when I mentioned it can reduce your flight test time in half. It can only reduce your test program if the engine and propeller combination have been approved by the FAA. Since I live in the U.S. I really have no idea how this works in other countries so be sure to check the regulations in your country.
The kit manufacturer will probably recommend an engine and propeller combination for your kit. A deviation in engine horsepower and you will need a deviation from the propeller size and possibly the propeller profile. A propeller is specifically designed to stringent standards for a given horsepower, rpm, and set of performance expectations of the aircraft.
Good news and more good news. Stay with what the kit calls out and you will have no or few problems with the engine and propeller installation. If you have a passion to just do it your way, there are propeller manufacturers and engineering consultants who can help you.
Here are some sources for gyroplane propellers and also some custom help sources. When you are ready see Reduction Units.
Aero Composite Propellers www.AEROCOMPOSITES.com
MT Propellers www.Jimkimballenterprises.com
Prince P-Tip www.princeaircraft.com
Powerfin Propellers firstname.lastname@example.org
Vesta Propellers www.VestV8.com
Ed Sterba Aircraft Propellers Pho: 941-778-3103
Orion Technologies www.oriontechnologies.net