Short answers, pulled from the story.
Global warming potential (GWP) measures how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to the same mass of carbon dioxide, which is defined as having a GWP of 1. It is calculated as the time-integrated radiative forcing from an instantaneous release of 1 kilogram of the gas, divided by the same value for CO2 over a chosen time horizon, using figures compiled by the IPCC.
Methane has an atmospheric lifetime of about 12 years, so it decomposes relatively quickly compared to CO2. Over 20 years its GWP is around 81-83, but over 100 years it falls to roughly 27-30, and over 500 years to about 7-10, because much of the methane has already broken down through chemical reactions in the atmosphere by those later points.
Carbon dioxide equivalent (CO2e or CO2eq) converts the mass of any greenhouse gas into the equivalent mass of CO2 that would cause the same warming. It is calculated by multiplying the mass of the gas by its GWP. The unit provides a common scale for adding together climate impacts from different gases, and is expressed globally in gigatonnes.
The Kyoto Protocol originally required GWP values from the IPCC Second Assessment Report. This was updated in 2013 at the Warsaw UNFCCC meeting to require 100-year GWP values from the IPCC Fourth Assessment Report, published in 2007. The Kigali Amendment to the Montreal Protocol also requires GWP100 values from the Fourth Assessment Report for hydrofluorocarbon phase-downs.
Sulfur hexafluoride has a GWP of about 17,500 over 20 years and about 23,500 over 100 years, rising further to about 32,600 over 500 years. Its atmospheric lifetime is approximately 3,200 years, which is why its warming potential increases at longer time horizons rather than declining.
GWP is defined on the basis of emissions, and human-generated water vapour from sources such as cooling towers and irrigation is removed by precipitation within weeks. This means it cannot accumulate in the atmosphere, giving it a 100-year GWP estimated between -0.001 and 0.0005 despite water vapour having broader and more potent infrared absorption bands than CO2.