I think this is how it works.
GFHi is the maximum allowable overpressure on surfacing, as a percentage of the difference between the ambient pressure line and the M value line.
So during the dive (at least for Shearwater computers), the computer is constantly calculating how long it will be - if you remain at the current depth and the current mix - before a direct ascent to the surface results in surfacing at GFHi.
@tursiops posted this in the GFlo thread, along with a link to the Shearwater page:
On a NDL dive, which computers' NDLs are not affected by GFLo?
So for example, you are diving 30/70. The computer determines that in 20 minutes, if you stay at the current depth without changing your breathing mix, and then ascend directly at a safe ascent rate (specified in the algorithm), you will surface with your leading compartment being at GF 70, and it will display NDL=20. If you switch to 40/85, then the computer determines that if you stay for 25 minutes and surface, your leading compartment will be at GF85. If you stay long enough so that a direct ascent will result in a GF higher than GFHi, then your computer will start generating deco stops.