Bti. The brand is mosquito bits by summit in my region.
Sticky traps will help but will never handle the problem. The mature gnats are annoying but don't cause damage like the larva do. Bti will kil the larva in the substrate. It'll take 2 weeks or more to fully eliminate the problem. Use it preemptively and never have the problem again.
Gnats often come with the substrate from the store you bought it from. Too often that bulky stuff is stored outside and it picks up gnats and whatever else.
apply every 2-3 weeks from get-go... Never have a gnat issue. Might see 1 or 2 occasionally. That tells you the substrate definitely had some in it before you got it. They damage roots and the roots are the most important thing... so.. save the roots!