The most logical transition to complete the text is "Consequently,".
The revised sentence would read:
"In recent years, some economists have argued that gross domestic product (GDP) is an inadequate measure of a nation’s wellbeing, as it fails to account for factors such as income inequality and environmental degradation. Consequently, they have advocated the use of alternative metrics like the Genuine Progress Indicator (GPI), which adjusts GDP by factoring in the costs of crime, pollution, resource depletion, and other negative effects related to economic activity."
This transition indicates a cause-and-effect relationship between the inadequacies of GDP and the suggestion of alternative metrics.