I never had gnats until this year. I have not changed a thing in my house and they are not in the greenhouse that I can see, but the doors and windows and vents are all open so they could be just flying away. I use the sticky papers plus you can use the Apple Cider Vinegar in a small container like you do for fruit flies and see if it works. I do know they seem to love water. They even get in the dogs water dish in the house. Not sure what happened this year to have such an explosion of gnats, but whatever it was it affected all of us.
You need to treat your plants a few times before you bring them inside. I seldom have a problem with the jasmine and the camellia until about November or December and then the camellia gets the scale ...last year, it did not get it and I think it was because I washed it good and sprayed it several times before bringing it inside. I also trim the jasmine before they come in. The Mandevillea also gets trimmed as does almost everyone that comes inside. I also treat the soil before bringing them inside with same insect spray.
