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How did the Gelasian Stage become the first stage of the Pleistocene Epoch?

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Map showing the extent of the Laurentide, Cordilleran, and Greenland ice sheets during the Wisconsin glaciation, roughly 100,000 to 11,000 years ago. The Laurentide Ice Sheet covers most of present-day Canada and parts of the northern United States in light blue, the Cordilleran Ice Sheet covers the Pacific Northwest in teal, and the Greenland Ice Sheet is marked in purple. Modern international and state or provincial boundaries are outlined for reference. — Pleistocene Epoch Ice sheets during the Wisconsin glacial episode penetrated deep into North America.

The Gelasian Stage became the first stage of the Pleistocene Epoch following a 2009 ratification by the International Union of Geological Sciences (IUGS), which accepted a proposal from the International Commission on Stratigraphy (ICS) to redefine the base of the epoch.12

Previously, the Gelasian was considered the final stage of the Pliocene Series, and the Pleistocene was demarcated by the base of the Calabrian Stage. However, many geologists argued that the base of the Pleistocene should be moved to an earlier time to better align with evidence of Cenozoic glaciation and regional climate changes, such as those observed in Chinese loess deposits.12

The new boundary was established at the Global Stratotype Section and Point (GSSP) at Monte San Nicola near Gela, Sicily. This marker is dated to 2,588,000 years ago, placing it within 20,000 years of the Gauss-Matuyama geomagnetic reversal, which allows for more effective global correlation.12