The GF99 value shows the current leading tissue inert gas super-saturation percent gradient as defined by the Bühlmann ZHL-16C decompression model. If that sounds confusing, then recommended reading is Erik Baker’s paper, “Clearing Up Confusing About Deep Stops”.
This value can be used as a rough measure of the current decompression risk. A value of 100% (Bühlmann’s originally allowed super-saturation limit) is now generally accepted to be too risky. The GF low and high conservatism settings are used to scale Bühlmann’s original limits to reduce risk of decompression sickness. The default GF low value of 30% limits the super-saturation gradient at the deepest stop. The default GF high value of 80% sets the surfacing limit. Between the deepest stop and the surface the GF limit is linearly interpolated.
The GF99 value shows the current super-saturation as a percentage of Bühlmann’s original limits. If no super-saturation exists, then the GF99 displays “On Gas”. Once a super-saturation exists, the GF99 is displayed in green. It switches to yellow when the GF99 exceeds the value defined by the GF conservatism settings. Above 100% it is displayed in flashing red.
In extreme conditions, decompression risk can be traded for operational risk by using the GF99 display. For example, in a low-gas situation or perhaps a bailout or injury, you may want to get to the surface faster, but not blow off all the remaining decompression stops. You could then violate the stop displayed by the Predator, and decide to follow a more aggressive GF99, say 95%. This will get you out of the water faster, at a higher risk of decompression sickness, while still remaining within some limits. Upon resurfacing, appropriate action for omitted decompression should be followed such as rest, surface O2, monitoring symptoms and contacting DAN or other diving medicine center.
Conversely, the GF99 value can also be used to add conservatism. For example, if you knew that you had been exposed to factors that increase decompression sickness risk such as exercising during the bottom time or being colder on deco than the bottom, then you could use this value to extend shallow stops. Of course, you could always just add a few minutes to your last stop, but this gives a quantitative measure to correlate with “how you feel” after the dive.