Gradient Factors can be made to sound very complicated but are just two numbers, a high gradient factor (GF) and a low gradient factor.
Both are expressed as percentages and they represent a percentage of the way towards the m-value*. 30/80 are popular gradient factors at the time of writing. In this case the low GF is 30% of the way to the m-value while the high GF is 80% of the way to the m-value.
This means that the first deep stop will be introduced at 30% of the way to the m-value rather than at the m-value itself or, to put it another way, 100% of the m-value. Similarly, a high GF of 80 means that the final stop will not be cleared until the diver is at 80% of the m-value, rather than 100% of the m-value.
This means that the low GF controls the depth of the first stop, while the high GF controls the length of the last stop.
*m-value – The term “M-value” was coined by Robert D. Workman in the mid-1960’s when he was doing decompression research for the U.S. Navy Experimental Diving Unit (NEDU). Workman was a medical doctor with the rank of Captain in the Medical Corps of the U.S. Navy. The “M” in M-value stands for “Maximum.” For a given ambient pressure, an M-value is defined as the maximum value of inert gas pressure (absolute) that a hypothetical “tissue” compartment can “tolerate” without presenting overt symptoms of decompression sickness (DCS). M-values are representative limits for the tolerated gradient between inert gas pressure and ambient pressure in each compartment. Other terms used for M-values are “limits for tolerated overpressure,” “critical tensions,” and “supersaturation limits.” The term M-value is commonly used by decompression modelers.