I would like to offer some information that may help our understanding of GF99 and SurGF. This may be old information to those of you who have been using GF's. Here are the two equations used to calculate GF99 and SurGF (they may not be exactly the equations Shearwater uses but they serve here to illustrate the concept):
GF99 = (p_ctc - p_amb) / (p_mv - p_amb) x 100
SurGF = (p_ctc - sp) / (p_mv - sp) x 100
where:
GF99 = % of the m_value at the current depth as measured from ambient pressure.
SurGF = % of the m_value at the current depth as measured from the surface pressure.
p_ctc = pressure in the controlling tissue compartment.
p_amb = ambient pressure.
p_mv = m_value pressure at the current depth.
sp = surface pressure.
The definition as given by Shearwater and mentioned in the article sited at the beginning of this thread is that SurGF is the value of GF if you could instantaneously surface from your current depth. The key word here is instantaneously. Obviously, you can't surface instantaneously so the value you read at depth is not the GF you'll see when surfacing. As you ascend your tissues will be off gassing which will reduce your SurGF when you eventually get to the surface. So, it represents a worst case value of the controlling tissue compartment (CTC) at the depth and time you look at it. As you ascend it will slowly decrease because the pressure in the CTC is off gassing and therefore the numerator in the equation above is decreasing faster than the base (p_mv - sp). SurGF should always be higher than GF99 because the sp is always lower than the ambient pressure during the dive.
The opposite is happening with GF99. It is increasing as you go shallower. Because of the half-time of the CTC p_ctc is not decreasing as fast as the ambient pressure. This suggests that GF99 and SurGF are converging as the diver ascends from depth. In fact, this is exactly what happens because when the diver surfaces, p_amb in the equation for GF99 becomes the sp and both equations must yield the same result.
Another thing I would like to point out is that in the equation for GF99 in order for the numerator to be positive, the p_ctc must be higher than p_amb. This implies that the CTC is off gassing. When the p_ctc is less than ambient pressure, the numerator being negative, the CTC is on gassing.
Since SurGF is always measured against the surface pressure it becomes an indication of how much inert gas the CTC contains. This is why you'll see a value for SurGF while GF99 reads on gas. GF99 is measured against ambient pressure and is indicative of the rate of off gassing. The higher the GF99 the higher amount of gas in the CTC and the higher the rate.
Edit 1: added definition for sp and cleaned up the last sentence.
Edit 2: added last paragraph.