Well, just scroll down a bit and see military production numbers on that Wikipedia page. The US produced over 100,000 tanks compared to German's 67,000 and Japan's 3,700 (although the Soviet Union slightly outweighs the US here), 250,000 artillery pieces compared to 160,000 for Germany and 13,350 for Japan (the USSR once again outweighs the US here, with over 500,000), about 2.7 million machine guns compared to about a million total for Germany and Japan combined, and then the real kicker: 2.4 million military trucks, compared to 345,000 for Germany, 166,000 for Japan, and 197,000 for the USSR. Hit up the page on Lend-Lease, and it claims that American trucks made up about 2/3rds of the total truck transport for the Red Army, in addition to 2,000 train locomotives provided (no clue about how that relates to the total though).

You're right, of course, that GDP isn't by itself a good measure of war-fighting ability, but I think the US did as good a job as anyone at throwing the entire country behind that particular war.

As for war-winning magnitude, it's always hard to say, of course. On the one hand, it's hard to imagine Germany properly conquering a space as vast as the USSR. On the other hand, they sure came close to taking a lot of important places, like Moscow and Leningrad. Logistics is what wins wars, and the Soviets needed it even more than usual with their massive relocation of industry away from the invasion, and of Siberian military forces toward it.

In any case, Stalin seems to have thought that the American contribution was essential:

"Without American production the United Nations [the Allies] could never have won the war."

Well, he said it. Who knows what he really thought. I wouldn't necessarily put a lot of stock in his stated opinion on the matter, but I thought it was an interesting quote anyway.