Why Are Little Gnat Like Flies Coming Out Of My Kitchen Sink Drain?

  That is just so gross.But you're not alone.  "Drain flies" they're called.  And they're surprisingly common. See picture here.

  The live on the organic matter in the "trap" underneath the sink drain pipe (the you-bend).

  Basically, to get rid of them you've got to remove their food source.  Which means clearing out the place in the drain where they're breeding. The usual solution is a heavy duty acid, usually containing the following: Hypochlorous acid, sodium salt and sodium hypochlorite.  The most recognised brand name would be "Drano".

  Drain cleaners are seriously nasty stuff to all life forms, so be very careful how you handle it.Eco-friendlier alternative is to take the sink apart and manually clear the drain.  This is actually more effective than the chemicals, although (obviously) more revolting, too.

  In the future try to throw food waste in the rubbish bin, not let it go down the sink.  This is kinder on sewage systems, anyway.

I have these little flies too. I've seen them mostly in my kitchen but also in my bathroom!! I poured a lot of bleach down the drains in my house and let it set for a long while and it seemed to help but a few days later, they were back. It's disgusting! I am a very clean person and I am stumped on how to get rid of them completelly although I am going to go buy some drano type stuff today and  use it.  It seems like every fall I get these and they are usually gone by now! I am praying that the drano works.  A friend told me to get this drain stuff called "the works" because it is so very potent! They sell it at dollar general so that's what I'm going to try to get. She said to even pour it down the drain to the bath tub.  I'm going to try it!  If you want to know the out come, just let me know!!  Good Luck!

Fruit Flies - they are called fruit flies and are attracted to the sink area because of the moisture, there are reminants of food particles, or anything else that goes down the sink - if you leave any moist and especially sweet foods on your counter, you'll find them there too.
The only real means of getting rid of them is to completely clean and disinfect the area and keep it clean.
(they've been known to hang out inside garbage cans too)

Windex or its equivalent. Several years ago my wife squirted some window cleaner down all of our kitchen drains (in desperation) after we had gotten rid of most of the previous batch's brood. She sprayed about 10 squirts into each of them 3-4 times per day for the next week or so - no new gnats. We thought we were rid of them - she stopped spraying. 5 days later a new batch arrived. We started spraying the drains every day or 2 through the remainder of the summer and the gnat problem was gone. The convincing part of my argument is that for the past 6 years I keep a bottle of generic window spray on hand at all times and hit the drains a couple of times per week during warmer months, and I haven't had a single reinfestation but for the time I was out of town for 2 weeks (and lemme tell you that was pleasant to turn the lights on to when I got home...)

Dump a little bit of bleach in your drain and you can also put a night light in an outlet close  to the sink with 2 sided tape around it to get rid of the ones that are airborne.Someone once told me to do the night light with a dixie cup of water and fruit juice an they will drown in it,but the bleach and tape work well.

Is it possible for gnat to travel up the drain hole next to my furnace and water heater and be blown out of my heat vents or come up my sink drains

You should try to talk to an exterminator so that he could exterminate your house and maybe they would leave

Make sure you have a flap on your pipe coming into your house that is put on properly make sure it is not backwards