GDP = (C + I + G) + (X - M), total consumption plus total investment plus total government consumption, plus the balance of trade... M is also counted in C, I and G. "Total consumption" means the consumption of domestic goods and the consumption of imported goods. Same for investment and government consumption.
The "- M" component is just there so that things which have been measured as counting towards GDP, but also count the consumption/investment/etc of foreign goods, have the "foreign goods" part removed... all that stuff is present in C, I and G. So we subtract M from everything so that we don't count foreign made stuff.
what if X=0, GDP=0; then M=CIG,
i.e. "everything consumed domestically was produced foreignly" ??
you're saying, that "M is already buried within CIG" ??
I.e. de facto, "government accountants" hawk over every domestic market; over the course of the year, they tabulate all "CIG" expenditures occurring in the country; meanwhile, "customs accountants" hawk over every domestic port-of-entry; over the course of the year, they tabulate all "X-M" entering & leaving the country. Then, the former "CIG" grand-total includes both domestically, and foreignly, produced Goods & Services; subtracting off "-M" yields that part of "CIG" which was produced domestically. And then, you add in "+X", figures given from foreign markets, wherein domestically-produced products were sold, "out-of-sight of the government accountants above".
so, "all else equal", reducing iMports would leave gDp unchanged -- foreign products would vanish from domestic markets, the "government accounts" wouldn't see them for sale, reducing "CIG"; the "customs accountants" wouldn't see them entering the country, reducing "-M" by the
exact-same amount. Logically,
Domestic production is not (directly) related to Foreign production; Foreign products could be excluded completely, with no (direct) impacts on Domestic activities. However, shortages of previously-Foreign-supplied products,
i.e. Imports, could plausibly impair Domestic production, which was dependent upon those Foreign parts, resulting in (indirect) reduction to GDP. If so, then denial of access, to superior Foreign parts,
i.e. lower Imports, could actually reduce GDP !!
(Cp. Libertarian laissez-faire -- if foreign parts are economically superior, then 'tis better to use them; cp. "cheaper foreign labor")
ultimately, the only thing that increases GDP, is a vigorous domestic "DIY" mentality --
i.e. "work more, for less",
i.e. "be more economically competitive", so that domestic products are preferred
we need to borrow money from abroad in order to buy goods from abroad.
"foreign financed Imports"
what if all earth utilized a single "universal" currency -- some sort of "electronic gold", according to which, all national currencies would be "automatically inter-convertible" (as if they were all backed by 'phantom electronic gold') ?
then, purchases made in US dollars would be legally equivalent, to those made in (a different number of) Yuan. Perhaps, instead of a "gold standard", there could be a "real absolute value standard", whereby currencies could always be converted, on demand, to an appropriate amount of "absolute value", not necessarily all in gold, at then-current exchange rates
("i demand to convert my money", "sure, here's 1ton of gold, 2tons of silver, 4-truck-loads of timber, a flock of sheep, a thousand bushels of corn, and a partridge in a pear tree")