Those gnats are fungus gnats that thrive in moist soil. You may be overwatering. Also, your soil looks like regular potting soil and if so, that may contribute to your overwatering problem because it retains to much water.

Check to see if it needs water by digging
a couple of inches into the soil mix to see if it is damp. Alternatively, shove
a wooden skewer halfway between the tree and the wall of the pot and all the
way to the bottom. Pull it out and if you see particles on it, then it does not
need watering.

Most eople on this forum use either the gritty
or 5-1-1 mix. The ingredients for 5-1-1 are easier to find. This is what I use:

My version of Al Tapla's 5-1-1 Mix

· 5
parts pine bark mulch (remove ½” pieces through 1/2 inch screen)

(Some people use Repti Bark instead of
mulch. They do not screen it. )
Be sure to soak the bark overnight.

· 1
part sphagnum peat

· 1
part perlite (coarse, if you can get it)

· 1 Tb garden
lime /gal 5-1-1 mix (measure the 5-1-1 mix)

· Osmocote
plus (follow manufacturer’s instructions). This is optional.

·
Moisten well and wait at least 2 days before using

Will the lil guy come out of this lead dropping thing? I hope he's not toast. In in Kansas... And the weather has been awful.. One day hot one day cold. I'm chasing the sun with the poor guy. I had him set in one spot since I brought him in and when he started dropping I've been moving him around. Is there a spray I can kill those gnats with?? Thank you again!

It's tough to say, but long-term survival is unlikely if he remains in standard peat-based potting soil. Chasing the sun shouldn't be necessary (and it sounds like a lot of work!) if the plant gets around 6 hours of sunlight a day, you're probably fine. Lemons in particular drop their leaves when light levels change too quickly anyway so it's probably better to leave it be. If I were you, I'd:

1. Set the tree somewhere that gets 6+ hours of sunlight. If it doesn't, see about potentially adding extra light through the use of a grow-light of some kind. Temperatures should stay above 55F if possible.

2. Only water the tree when it's completely dry and the pot feels lighter, until you can repot into a better mix.

3. Repot into a better mix, being sure to wash ALL of the old soil off the roots and trimming away any mushy, soggy, smelly, or dead roots. Try to use a container that is just slightly larger than the size of the rootball - you want to give room for the roots to expand but you don't want them swimming in soggy soil either.

4. Let the tree recover. Don't fertilize, and water only when the plant is dry - Vladimir's wooden skewer trick is great for this. The plant will be working furiously on growing healthy roots.

Eventually, you may see a flush of blooms leading to new leaves or new leaves on their own. This is when you can begin feeding the tree fertilizer - most of us on this forum use Foliage Pro 9-3-6 coupled w/ Osmocote Plus slow-release fertilizer.

Oh, and don't worry about the gnats. They should die out once the soil is suitably dry, and they don't hurt the plant - they're actually a good marker that the soil is too damp.

Hope this helps, and good luck!

I would add supplemental lighting in addition to figuring out a good watering schedule (not too much/often) re-potting into a better soil will certainly help but I am not sure on the best time to do it. For lighting I copied Laura's set up and I picked up some cheap CFLs off of amazon and put it into a cheap ikea floor lamp and leave them running 10hrs a day about 4-6 inches from the plants.

https://www.amazon.com/gp/product/B00M6SR1JM/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B007RKKEHA/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

http://www.ikea.com/us/en/catalog/products/20110903/

65 degrees today so I sat my little baby outside!! Thank you all for the help!

Lovely green tree against the grey background. He is happy!

Finally another sunny day!! He has some new growth!! I have hope!!

Things are looking up!

...and now the lovely kansas weather is back to cold. Can't go outside with this baby. Wait for another warm snap. But still getting new growth!

You have scale. Read my reply to your other post.

Haven't repotted yet. But have tried all the other hints. If this stupid weather would straighten out it would help! But we've got some new growth! And stopped dropping leaves!

Looks good, keep us updated!