Yep, maybe GDP is not a great measure if one country can produce a box of cookies for $1 while another $4. Turns out physical output is important (bushels of wheat, pounds of steel, # of cars, etc).
As we're seeing now, it's an even bigger problem when a country produces an advanced military gadget for $1 and the other country can't match it in terms of production for even $50.
That's why difference exists in the first place - because of power imbalance driven by different levels of technological progress.