I was wondering if you put a reasonable thick layer of sand on the top of the soil ( indoor pot),
wouldn't that be impossible for the gnats to pass through the sand since the sand is too heavy for them?

Gnats can even burrow thru sand.  Unfortunately

There are members who have tried it. I did it too but I don't think I had any gnats to give it a whirl. Not that I'm complaining. I put sand on top and then a nylon stocking over the cup to keep any hatched gnats from exiting the soil. I was told the sand has to be dry or they can burrow through it, but then you can't water. 

Maybe someone else who tried the sand can offer their experience.

I had a lot of gnats before. Mosquito dunk works for me.

I have tried using sands this winter. It really helped me reducing the number of gnats. The key is to keep the sand dry.  You will not 100% eliminate the gnats by only using sands. Gnats love to lay egg in moist soil/medium. Dry sands discourages them from laying their eggs there. I like to sprinkle some mosquito dunks on the soil, water it, and then pour some sand over it. So far no more gnats around. I don't normally water my cuttings until I'm ready to move them to 1 gallon pots. When I'm ready to move them, I will sprinkle some more mosquito dunks over the soil in the gallon pot and water them.

I used sand on the surface of soil on a few 1 gallon pots combined with pantyhose to cover the drainage holes:

http://figs4funforum.websitetoolbox.com/post/Office-window-figs-fungus-gnat-control-6184249

This solved the gnat problem for those potted figs but is not really practical to scale this up to protect 30-40 rooted cuttings in cups.

Back in Oct I had about 30-40 flying around at any given moment. Haven’t seen a gnat in months. Sand worked for me but its a PITA. My sand is usually wet, when its dry my plants need water. I use about a quarter inch on top and restrict access to the drainage holes. No Chemicals were used.