I have a composite decking floor in a garden shed fixed to steel, even out of direct sun and most of the weather it still contracts and expands.
While the deck jig puts the boards in nicely and the screws have a good hold and I think my shed would stand up to having something blocking the expansion gaps, I would not be too confident if it was out in the elements.
I have used some as a screen that floats in a frame with spacers, it is on the shady side of a tree and when does it ever rain in Australia? Fairly short span too but it has stayed straight, an offcut has been out in the weather and that is a bit banana shaped now.
The recommended gaps would be for a full length, a shorter length may get away with a smaller gap and there may be a margin of safety in the recommendations. No gaps has never worked from what I have seen once it has been exposed to a weather cycle.
Are you sure it is the composite that's moving? Steel moves with temperature change very quickly.