I think we are talking two different things here, like Zoomer I run water over the pave walkway in the greenhouse for humidity when the weather is hot, I also keep several cans full of water warming up to water the plants next day. That is also for humidity which is needed for most greenhouse plants.
Now the gravel trays, the idea is to have the gravel damp so it is humid around the tomato's which they need. I would suggest taking a trowel and turning the gravel round the pots every couple of days, this will disturb the growth which will be harmless but unsightly, bringing fresh gravel to the top and let it dry out a little by keeping the water low in the gravel trays.
My tomato pots sit on a bed of gravel which I take the rake to a couple of times a week disturbing the small seedlings that seem to want to grow and clearing any weeds, chick weed seems to love greenhouse gravel, a run over with the Dutch hoe sorts that out.
Air movement is essential in a greenhouse so I have extra vents which I can open when needed and on very hot days (if ever) I use a fan to move the air.
Hope this will help but you will always get moss either on pots or the gravel when you have humid warm air.