I’ve read that the larvae could kill seedlings, first I’ve seen it, sorry for your loss.
My take is the same as @buzzmobile said except I used ground up mosquito dunks.
About 0.2 gm worked into the top inch of soil per 5 gal pot worked like a charm for me. Plan on re-dosing every 10 to 14 days as the BTi does not seem to persist in the soil on its own.
My variation on @Upstate is I buy 6 mo. ahead and let the bags ‘age’ in the garage, summer cooks them and winter freezes them. That gets about 95% of gnates and the very few that manage to survive that are super weak and feeble & diatomaceous earth is great too.
Cheers
G