Bti - in north america the brand is summit and it's called mosquito bits or mosquito dunks - slgithtly different use.
While it is true that the adult gnats don't cause damage, the larva eat your roots. They are absolutely something you should worry about. It will take a couple weeks for something like Bti to get control of it.
you can use bti pre-emptively. Gants often come with the soil from the store. I use bti the first month or so. I only stop if i don't see any gnats.. even if i see 1 or 2, i continue the use. Never have a gnat problem. Easily avoided and it's relatively cheap to do.
If you water to frequently, this will exacerbate the problem. A good wet-dry cycle of top soil helps avoid it, but when they are infested at the store or during transportation, there's no avoiding it with good watering habits..
sticky traps can kill off the adults... pyrethrines can kill on contact, but don't use that or basically anything else in flower. bti is safe in flower it's just a top-dress you water-in or a dunk in your reservoir. it is a bacteria. It different subspecies has been used on teh world's food crops for a century, so it is safe.