Keeping a small metal shed dry.
I agree; the first thing that I would do is to ventilate it, regardless of anything else. Then, I might add some rigid insulation, at least to the walls that receive morning sun, and the ceiling, in order to reduce the rate of temperature changes within. Depending on how it is constructed, you may be able to notch the sheets such that you can slip them in place, and not even have to otherwise attach them. And, I would probably lay a vapor barrier, i.e. plastic sheeting on the gravel floor, on which you could then lay some planks, or even just pallets, on which to walk. But, not knowing the conditions, hence whether something on the floor will help or not, you might first lay a large garbage bag out on the floor, and check it the next morning, to see which side of it is wet. I would think that should provide an indication of whether moisture is coming through the gravel as well, or if all of it is coming from the air within.