I second the Gnatrol off Amazon.  Use it weekly to break the cycle.  I just used a spray bottle to moisten the top 1 1/2 to 2 inches of the soil.  Worked like a charm.
You can reuse the soil.  Just nuke the tar out of the soil in the microwave.  The soil you purchase can even come infested with gnat eggs from the get go.  So you could even just routinely nuke the soil before using.
Good luck!

I used a combination of gnatrol and carniverous plants this past spring to keep the gnats from doing too much damage.  But Lukie, this time of year with all the humidity outside wouldn't it make more sense to do your propagation (post-rooting) outdoors?  For whatever reason gnats don't seem to be a problem once the cups/1 gallon pots are moved outdoors.  That's the beauty of propagating from cuttings this time of year. If it were me I would set up the cuttings in cups outside (in a shady area protected from rain) once they have rooted.

You know rewton, that has occured to me and I think that will be the route i take. I just put a bunch of new cuttings in moss, in a month or so when i cup them i will put them outdoors.

I also had tremendous root development using the DNG.  Some of the cuttings would start to leaf out and others would just barely start to show some buds.  Then no matter what I tried (dome cups, large containers with lids, little water, moderate water, etc, etc), most would die.  I was losing over 90%.  Went back to tried/true methods without DNG and seems to be working much better.  My hypothesis is that the DNG while producing outstanding roots in a short time, all the energy is zapped from the cutting leaving nothing to continue on.  Just a theory....

I use both methods, bug zapper for adults, and mosquito dunk for larvae.
I crushed dunk and mix it with water to water the plants. The bacteria will stay for months, even the adult lay eggs, their larvae will die. You will see the entire population almost wipe out in a couple of weeks.
I had a lot of fungus gnats earlier this year. My bug zapper was totally covered with the dead gnats in a few days. After I apply mosquito dunk, they are gone. 

Mosquito dunks work but are very very hard to get to break down from their shipped form - a lot to say just because I forgot how to spell desolve and am too lazy to open another window and  . . . anyway --  and they clog up stuff like my watering can . . . so -- I use Gnatrol WDG. It's 37.4% BT. I buy on Amazon and really like the results as I have been cursed by gnats. I also use the yellow sticky paper and enjoy counting the dead little b#@^&@ds every day. After using Gnatrol though my gnats have gone from flocking to my laptop screen at night to RIP.
I used to use mosquito dunks and they certainly work but I think Gnatrol is a better solution.

Hi Michael:  dissolve is spelled thus.  How catchy (and ironic) that you included an admission about spelling dissolve and yet still ended your post with the word "solution"!

Hope all's well with you in VA.  

Mike   central NY state, zone 5a

<edit> P.S.  Someone once told me "Solution this", and I asked him what he wanted me to dissolve it in.  (I think he meant "solve this").