Hello
I think every batch of seeds sometimes has some runts. With those being a bit behind you might want to water them a little lighter than the others until they catch up so they're not getting drowned while developing more roots and mass.
With the gnats.... I've been battling those this year and I've definitely had a few babies really struggle...even die, until i got it under control. If you're able to go by a garden store or something like that and buy "mosquito bites" they're good, sprinkle them everywhere and even mash a bunch up and leave in water for 18-24 hours and then water all pots with that water. If you can find neem meal that's also a very good one to use and do the same thing, put in with a gallon of water let sit and stir occasionally over 18-24 hrs and then soak all the medium and it should kill any larvae. Treating a couple times is a good idea too, not just once.