The Green Travel Carbon App was on Play since 2011 and was updated in 2021 to a complete app. The 2.0.8 level adds background execution which completes the functional baseline. Sources are maintained and freely available in my forgejo git repo for this and active development moves into the Personal Carbon Accounting app in my PMTA product line, a generic base for creating personal motion tracking apps.
From this point GT2 will be maintained to keep the stack current, track current Android support window.