Dam now I know a trick, unfortunately the like to vaca at my place too.... hate gnats the most out of all the bugsYou should be able to pick up some Mosquito Dunks at any home improvement store-I spend $8 for enough to last through an entire grow-They're way cheaper than gnat nix, evidently.
You do need to keep your plant water in a bucket or something if you use the Dunks- they need to soak in the water for 24 hrs. to populate the water with BTI.
It works very well,and doesn't hurt the plants at all,and if you use it all the time,the gnats will never come back.