Most composite deck manufacturers recommend use of a spacing tool when installing decking. Some even have their own tool just for this purpose. Since this solution is too late for you, I have one that is unconventional but effective.
Assuming that the deck boards have been put down straight, you can take a circular saw, set the blade depth to be just a hair more than the thickness of the decking, and run it right down the joint between each board. As a result, you should end up with a slot between the deck boards that is equal to the kerf (width) left behind by the saw blade.
The kerf will probably not be as wide as the decking manufacturer recommends but it will certainly be better than not having a space at all. The kerf could be widened with a second pass or the same result could be achieved using a router with a plunge bit the width of the desired space.
To keep the cuts nice and straight, I'd attach a straight piece of wood, perhaps cut out of plywood because it is dimensionally stable, to use as a "fence" for the circular saw to run against. You'd have to measure the off-set between the blade and the side of the base plate of the saw to figure out where to attach this but it would make the cuts easy to do once that is figured out. The result would be deck board spacing that is even and allows for drainage year round.