Political Economy is complex. What GDP aims to measure is overall production, it's an explanation of wealth, however it paints a very uneven picture as it overvalues financial transactions and undervalues raw productive capacity. As a consequence, imperialist countries are overvalued (like the US), while production-focused countries are under-represented.
GDP is often pushed by liberal, western countries as it makes them look good. A more honest look, however, is multi-faceted and takes into account other metrics like social services, which often run into negative "profitability" or break even, as they aren't producing for profit. There's also the fact that the US doesn't outproduce in non-physical goods either. The US certainly has popular media and software, but it doesn't have overwhelming productive capacity in these areas.
The point of focusing on BRICS is because if you remove the financialized, almost ficticious look at capital as displayed by GDP, BRICS is more economically strong and significant, and this better reflects the real world, not just US-based self-perception.
Germany is also imperialist. The countries that rely on imperialism have higher metrics by plundering the global south. It's kinda like looking at life expectancy of the rich vs the poor in any one country, the better metrics of the top come at the expense of the bottom.