GWP already takes into account half life. This has been thoroughly explored. Leaked methane is 11x worse than the co2 formed by burning it when half life is taken into account.

Also, our best estimate is that the single half-life for excess CO2 lies within the range of 19 to 49 years, with a reasonable average being 31 years.

https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...