Product details
4 sets Primary Gears for Hyperion DS20xSMD
"GP4" stands for "gear primary - 4 sets". Any metal-gear servo (made by any company) use a plastic primary gear - or set of gears - closest to the servo motor. This isolates the gear-train electrically, and also serves as a sacrifice gear (or gears) in a crash.
Many users asked for low-cost replacement sets of these primary gears, so that they don't need to spend the money on a full gear set just to replace the primary gear(s) for their metal-gear servo.