Hi or low GI, it's the carbs that aren't absorbed by the muscles or liver and stay in the bloodstream that make us fat. Whenever we eat carbs, they are released into the blood, and insulin is produced to take the sugars in the blood and drive it into muscles or the liver. Once liver and muscles are filled, only then does insulin drive the blood sugar into fat cells. It also happens when there is too much sugar in the blood for the muscles and liver to absorb at a given time. This occurs either when you eat excessive amounts of carbs, or when you eat high GI carbs. When you eat high GI carbs, the sugars get in the bloodstream at a rate faster than the muscles and liver can absorb them, so insulin has to turn the excess sugar into fat. Low GI carbs are released into the blood more slowly, therefore the muscles and liver can absorb them. But if the liver and muscles are already filled, there is nowhere for the carbs to go but to fat cells, even if they are low GI.
The thing is, after you work out, your muscles absorb the carbs at a faster rate than normal, so even if you eat high GI carbs, the muscles will have absorbed them before they are turned into fat, unless you absolutely gorge.