Gate can quickly cut out low hum and noise from the signal, cut reverb tails short or exaggerate the dynamics in a beat. The possibilities are... maybe not endless, but should cover your gating needs.
Gate has simple but powerful controls for tuning in the desired effect, and can easily be driven by an external sidechain signal. The gate can also be flipped around to make a signal duck under another:
Gate shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Gate is also available as a Rack Extension for Reason over at the Propellerhead shop.