It's in the wiki:
Gate/Expander (block) - Axe-Fx II Wiki
There's no way around it. For some tasks you have to invest some time into getting acquainted with the parameters. I hardly know them all. Guys like Javajunkie, Simeon, Bakerman are the real knowledge gurus here.
The Axe-Fx is a pro device. Just like a G-Force, Eventide etc. And that comes with deep editing possibilities and challenges. It's not Sony's My First Modeler. Some ask: Why this difficult? While others keep asking for more parameters and possibilities ... just check the Wishes subforum. Hard to satisfy everyone, for Fractal.
Onto the ISP and gate block.
The problem with lots of 19" gates (G-Major, Rocktron Hush etc.) is that they are placed after the preamp. This way they can silence the entire signal, but it makes them less able to detect the incoming signal correctly, because of the difference between clean and distorted input signals. This is why it's often necessary to use different gate settings for clean presets and dirty presets in those devices.
There are other noise gates which you can put between guitar and amp. While these detect the guitar signal better, they are less capable of getting rid of noise originating later in the chain.
I owned a G-Rack Pro. The great things about it (except for the ground loops), are its internal "vector something" technology, also present in the G-String pedal, and its clever use of signal detection. It comes first in the chain, letting it detect the guitar signal properly, not bothered by preamp gain. But it applies its gating after the amp stage, therefore eliminating all noise.
You can accomplish the same by using the Sidechain Select parameter. You need to use to the Gate block for this. Can't be done with the regular preset gate (Layout menu).
Place the Gate block in the grid. Try right before or right after the Amp block. Now set its Sidechain parameter (SCSEL) to Input. Now the Gate block will use the "global" signal at the start of the grid for best results, while muting all noise amplified through blocks in the chain.
The Low Cut/High Cut parameters probably perform the same task as the Filter control on the G-Rack Pro. But you probably don't need to adjust those.
Attack, Release and Hold are okay at default.
If the Gate block behaves anything like the preset Gate in the Layout menu, set Ratio to around 2.30.
Now adjust Threshold to where the gating is applied to your liking.
P.S. If you are going to use the Gate/Exp block, you might as well disable the preset Gate (Turn Threshold until it says Off). Saves a couple of CPU %.
