The FlySky GT3B is a very good radio to start out with, The stock firmware has some bugs, like the expo doesn't work correctly. So this lead to fellow hobbyists writing and using a new firmware for the GT3B. At first the firmware simply fixed the bugs, buts its grown over time to offer a vast array of new features, models, and enhanced functionality!
However, flashing a new firmware and soldering on circuit boards is very nerve racking for some people, so OverkillRC offers them pre-modified, ready to go out of the box.
This new firmware expands the capablities of the GT3B beyond what any other Pistol Grip style radio offers. The most notable of these is that it can control up to 8 channels independently. Each channel can be controlled in a wide variety of ways through the new key-mapping menu. Methods of controlling a channel can be momentary or non momentary 2 and 3 postion switches, semi-linear control with step rates, mixing to another channel, and through a feature callled Multi-Position. What Multi-Position does is provide up to 8 ACTUAL positions that can be cycled through in any order with a rocker switch (to cycle forwards and backwards) or simply a push button switch to cycle through one way only and it'll create a loop.
Also, the Key-Mapping menu not only lets a channel be mapped to a switch/key; It also allows most of the other settings to be adjusted as well on the fly with a switch. For instance, trim, exponentials, dual rates, etc can all be adjusted with a switch while you are driving.
Another very useful feature that is usually only found in high end model radios is mixing. The new firmware has a mixing menu for Dual ESC's and 4WS. These mixes can also be controlled via any key with a wide range of control options.
For a full list indepth description of all the new features. Please see the feature tab.