Turkey's GDP peaked at $950 billion in 2013, just before Erdogan became president. By 2017, it had slid to $850 billion, and after the intellectual purges, slid to just $766 billion in 2018. [1] People may argue from ideology for or against politics like these, but I can think of no clearer indication of these policies' failure than the impoverishing of a country. [1]: https://tradingeconomics.com/turkey/gdp
GDP is not a north star metric. GDP in the US is high and we have more suicides than ever. Many authoritarian (or especially populist) governments KNOW they will lower the GDP because by limiting low-wage labor immigration or focusing on nurturing a native culture or preserving the environment and not buying into a soulless consumerism/globalization they are creating a GDP cap. "growth for the sake of growth is the i…
More broadly, I think lots of nations today could maintain healthy economies with flat or shrinking national GDP, but those are countries which have little reason to expect 'natural' GDP increase without a growth ideology. Most obviously, post-industrial nations which can't rely on growing populations or rapid labor productivity boosts from urbanization and infrastructure investment: Sweden or the UK could plausibly operate at near-constant GDP, and Japan could even grow per capita GDP while shrinking national GDP. More subtly, nations which actively avoid pursuing the common arc of demographic transition, urbanization, and industrialization: Mongolia or Bhutan might try to take an entirely different path. And crucially, any of these nations would need to plan for the outcome: if Sweden 0% GDP growth without intending to, it would point to serious economic problems.
Given all of that, Turkey seems like a singularly bad candidate for falling total GDP. The population is still growing at 1.5% and labor productivity at 2%-3%, while experiencing urbanization and major infrastructure expansion. Crudely put, 1.015*1.025 ought to net Turkey 4% GDP growth without considering anything any other factors at all, so a substantial decline points to something going badly wrong.