MrStinkyanswered grow question 3 years ago Yes. That is the stuff you need but be sparing with your dressings as DE can be used for amending soil, too much can mess with your composition.
If you can, try getting a 'powder duster' to apply to the top of your soil.
Did you also get nematodes SF?
You can get them online, a egg powder that you water in and they will eat the gnat larvae ;)