The best way to get used to the gyro is use it first on a well known aircraft. Just go with Rate mode first to keep your life and understanding simple and enjoyable. If the model is a 70 mph model, like a smaller lighter warbird, and your flight controls are set for smooth flying (not outrageous throws) then you could probably get away with 100 percent. But when I say "probably" I mean it is all relative. Each model is different depending on how your flight controls are set up, how fast you fly and what type of model we are talking about. A fast model or a model that has high control throws on the other hand may need as little as 10 or 20 percent gain.
The reason you want to use it on a well known model is you will get used to using a gyro first and don;t want to worry about learning the model too at the same time. I assume this is your first gyro like this based on the setup questions. Nothing wrong with that, just allow yourself some short learning curve to get up to speed and it will happen fast, it's pretty intuitive.
On faster jets the gain needs to be lower in general, especially if the control throws are also be large for aggressive flight. On that 150 mph jet I mentioned, it's also is set up for sport flying and aggressive aerobatics. I take off and land at 35 percent gain and fly at max speed at 10 percent gain or less... and do most of my aerobatics at mid speed at about 25 percent. The effect is very good yet gracefully presents itself when needed without being too much of a worry of over correction from the gyro.
I started out "learning" what sorts of gains work with what types of planes and speeds by using the gyro via having it on a slider switch. I used the "up" position of the slider mean no gain (off) and "full down" on the slider mean full gain (relative full gain as limited by my programming). I limited the "gain" in the transmitter end point programming feature so that on the initial warbird, for example, full gain was 100 percent. On the faster aerobatic sport jet full gain was 30 percent. Then slider positions toward "off" were obviously less on a linear scale.
You explore the flight envelope of the plane by setting a gain and then aggressively "bumping" a flight control then neutralizing it to check the response of the plane to the upset. If the "bump" on aileron for example leads to a soft stop of the upset, you are good "for that speed on that plane" at "that gain", etc. If you get an oscillation after the bump upset test, IOW, the plane waggles a bit on that axis while correcting for the upset, your gain is "too high" on "that plane at that speed". Etc. If, while flying, your plane starts to oscilate on it's own for any reason, your gain is clearly too high for "that plane at that speed". You need to know how to stop this sort of uncommanded oscilation. The cure is throttle idle, pull up to bleed speed. Once flying safely turn off your gain to zero and then when you take a breath or two re set your gain to a lower setting.
After a few flights you can get a really good "approximation" of what gains will do what for what plane at what speeds.
After testing each plane - takes a couple flights to do it right - then you can take your gyro control off the slider and put it into a general radio mix. For example, gear down means full gain (let's say full gain on this plane is 40 percent, then gear down get's you full 40 percent gain). Gear up means mid gain (let's say 25 percent. Like a gain you'd like while scale flying a routine). Then if you have a flight model switch that softens up your controls a little or another switch that you can slave, that might set you up for "high speed". IOW, gain might go down to 10 percent, for example, in this "mode". There are a million ways to do it but that's how I have my gyros set up now that I tested the planes on the slider setup way first.
I don't want to write a novella about it but those are the main points.