What exactly Ryza is depends on what source you're looking at. A planet by this name has been described as a Jungle World once, too. It is important to keep in mind that with
40k, looking at multiple sources has a very high chance of yielding conflicting information when it comes to details.
(alternatively, there really
are two Ryzas - hey, it's the Imperium

)
That being said - going by Codex fluff, Forge Worlds are Mechanicus territory, and thus do not raise regiments for the Imperial Guard as normal planets do. However, they are a prime supplier of arms and equipment, and the Guard Codex mentions how Forge Worlds are often visited by troop transports that have just recently tithed personnel for service in the Guard, and then gather at a staging area nearby a Forge World where they take on additional arms and munitions.
Also, of course Forge Worlds raise their own PDF as any other world does, but these are called Skitarii or Tech Guard, operate under supervision of the Cult Mechanicus itself, and occasionally deploy offworld for duties such as escorting Explorator missions or providing protection for a Titan redeployment.