Gnats in your house come into your home from the outside.
When you have gnats coming into your house the gnats are seeking food and a warm area.
So if you have fruits and other foods sitting out on the counter the gnats can be attracted to the fruits and other food so make sure to put the fruits and food away so that the gnats will not be attracted to your house.
Besides just fruits and other foods the gnats are also attracted and lured to things such as food spillage, moist potting soil, over watered grass or plants, garbage cans, puddles in kitchen, condensation around windows and vents as well as leaky pipes under your sink.
You can get rid of and kill gnats with a mixture of apple cider vinegar, water, sugar and dish soap in a bowl or other container that will lure the gnats to the mixture and kill them.
You can also pour bleach down the sink drains which also gets rid of and kill gnats.
Use screens on your windows as well which can help prevent gnats from getting into your house and clean up any food or drink spillage and keep the rooms where the gnats are moisture free.