Cut off the snorkel on the air cleaner box, so it's about 2 inches long. Then run a piece of hose from the cut-off out over the top of or beside the radiator, where it can pull cold air. Make sure it's not pinched anywhere. Rubber radiator hose is best, if you can find some big enough, or use flex air cleaner hose. The GL1000 normally gets its intake air from the area above the engine and behind the radiator, where it's nice and hot.
Hot air is less dense, and decreases power.
By pulling cooler air from the outside world, you get a better cylinder fill and more power. It takes a smaller throttle opening to do the same amount of work, and fuel economy goes up. Properly placed, it will pull cooler air from outside at least. Also, the stock snorkel opening is too small. Compare it to the combined sizes of the carb throats.