Normalizing comparisons of corporations and countries
GDP is a flow (of goods and services) while market capitalization is a stock measure. So I agree, this is not a great comparison. If we want to compare stocks, we'd compare total enterprise value (sum of equity and debt) to national domestic assets. But that's imperfect because much of what makes a country rich is labor income and that's not going to be in domestic assets. So we'd like to add in some measure of human capital. That's going to be imperfect at best. So I prefer to compare flows. 

Consider how GDP is a value added measure. If you import steel for 150 and export cars you make out of that steel for 250 the GDP contribution is 100. Some of that 100 went to capital and some to labor. What's the value added of the corporation? Think of the company as "importing" the goods it buys and "exporting" the goods it sells. The difference is paid out to three sources, equity capital, debt capital, and and wages paid to labor. So to make a number comparable to GDP, add together corporate profits, net income expense, and labor costs. This is the value added of the firm and much more analogous to GDP than firm value.