Your question implies that there is no opportunity cost incurred as a result of the government spending hundreds of billions of dollars per year on jets, tanks, guns, bullets, contracts, etc.
PoloHoleSet's point about there being too many variables is probably the best kind of response you're going to get. Why? Because an economy has a quintillion moving parts. The military industrial complex is merely billions of moving parts.
When you consider the thousands or millions of jobs in the private sector that exist to keeping our military supplied, there are a lot of people who would be doing other things if that industry didn't exist. Like working in other industries, going back to college or becoming entrepreneurs.
So, lets say that in some alternate universe there weren't new jets, tanks, guns, bullets, contracts etc.. being made, but that money was being spent by the government on infrastructure, scientific research, space exploration, etc. Or even crazier, what if that expenditure was subtracted from our tax burden, what would the citizenry do with with all that available capital.
Imagine if instead of working from January to March (or May/June, depending on your tax bracket) to pay your income tax, that money was yours to keep and do with as you please. Think of the savings people could accumulate and use to start a business or invest in a startup or grow their business or buy a house... the possibilities are innumerable.
Not to be insulting, because I mean this with all due respect, but your question is a bit short-sighted. Your question is like a southern plantation owner in 1845 asking "if there weren't slaves in the field plowing and picking cotton, corn, etc.. how would this impact the economy/GDP?" It's hard to say exactly, but we do know that there are better, more ethical alternatives to utilizing slave labor. We know that actually paying workers becomes more profitable for business owners because you get more productivity out them as a result of it being a voluntary relationship.
The 19th century was a fantastic experiment in laissez faire free-market economics and it produced the greatest creation of wealth, improvements in standards of living and working conditions in human history. Likewise, perhaps freeing up the hundreds of billions in wealth that is currently being directed by the federal government would allow all the moving parts of the economy to make the most intelligent decisions possible as to where to allocate those funds.
When private profit is derived from state expenditure then incentives become aligned with the state... and there is nothing laissez faire about that.