Passive temperature-controlled packages can be broken down into two basic categories of components: insulation and refrigerants. The package needs to counteract the heat flow into or out of the package wall in such a way that it maintains the product temperature within the required range.
The gel pack acts as a refrigerant which absorbs or releases heat while maintaining a constant temperature. This occurs when the gel pack melts or freezes. When melting, the gel pack absorbs heat and keeps the package from warming. Conversely, when freezing, it releases heat and keeps the package from cooling. The role of the insulation is to slow down the flow of heat, decreasing the amount of heat that the gel packs need to absorb or release over a given time period.
That amount of heat is also dependent on the external temperatures the pack is exposed to. While the heat flow is slowed by the insulation, it also increases by the difference between the external temperature and the product temperature. In other words, the warmer it is outside, the more gel packs are needed to maintain the product temperature. Adding more gel packs increases the time it takes them to melt because the additional mass adds more ability to absorb or release heat.
If you keep the insulation the same and vary the number of refrigerant packs, you will see a similar variance in the performance. The difficult part is choosing the appropriate combination of refrigerant and insulation to provide the most optimal package from a cost and weight perspective, considering environmental temperatures.