First, the Pro Rack G is 2 units in 1. I had the Pro Rack G and sold it when i realize that it sucked my tone in the effect loop. After that i only use my gates in front so the Pro Rack G was useless for me.
The ISP product is a Downward Expander, so it does the same job but differently. It's more progressive with a ratio like a compressor.
The ZUUL is a Noise Gate (ON/OFF) like the NS2, but more transparent.
It really depend on your applications. The ZUUL is better if you use lots of gain and play very loud with a drummer, because the gate itself is more useful in that situation. You can be more aggressive with your settings without affecting your tone.
Both are the best units at what they do.
Hope it helps!