I would vacuum the furniture thoroughly first and then use this method to kill them.
Are you sure they are gnats and not bed bugs?
Are they biting you? Are you sure they are gnats, usually gnats do not have around furniture.
Good advice from Cheryl. I’ve never known of gnats in furniture, but if it’s been outside, I wouldn’t be surprised. If you can kill one, look online or ask a bug pro, because it might be a termite, flea, or some other flying insect. You still need to vacuum and clean, but you may need to treat other nearby pieces, also. And then the vacuum cleaner and trash the cleaning cloths. Jewell