Beautiful pond and property!
This is supposed to work and it does indoors as I have tried it but for outdoors you would need quite a few containers or maybe just one big one near your pond:
Make an attractant by mixing together 2 tablespoons of apple cider vinegar, 2 cups of water and 2 tablespoons of sugar. Stir in a few drops of dish washing liquid and pour the solution into several jars. Set the jars in places where the gnats are congregating, indoors or out, such as near your garden or on the kitchen windowsill. Leave the traps in place as long as necessary to attract and kill the gnats.