You won't get rid of them like fruit flies. They eat the vermicompost and lay eggs in it. I just beat my gnats in my worm inn as of yesterday, but I'm still on guard for their return. It took about 2 weeks. I took a paper tray and filled it quarter way with with apple cider vinegar + soap mix as a fly trap. I added about 10 inches of shredded newspaper bedding to allow the newborn gnats out but not back into the compost. I left a light on above the trap to help them find their way out of the bedding and to force them to congregate. My worm inn has a mesh cover, so it was perfect for me to just blow 20-50 flies right into the trap through the mesh. I put the trap on top the bedding, and every few hours I'd tap around the sides of the bin to force the gnats to take flight - that's when they end up landing in the fly trap. When I went to feed the worms, I'd block all air ventilation except a very small inlet, and put a vacuum on the other side. The reduced inlet means lots of air pressure forcing any gnats in the air right into the vacuum. I'd let that run for a minute, and tap on the sides to suck up as many as I could. I'd disconnect the vacuum and leave it running for when I opened the container, I'd vacuum up the gnats as they tried to escape.

After this whole fiasco I decided to process our scraps a little better. I bought a second hand jar for our blendtec blender, and every few days I grind up a few pounds of scraps, put them into deli buckets and freeze them. Faster worm eating, less chance of flies. Freezing the food, far less chance of flies. I cover the deli buckets with paint strainers as they thaw to prevent flies at that point, then flip it upside down and use the paint strainers to squeeze the excess liquid out of the food. I bury it an inch below the surface, and make absolutely sure to maintain a LOT of bedding on top of the compost level. I will likely leave a jar with apple cider vinegar + soap indefinitely inside the composer, to serve as an early warning and hopefully nip any populations in the bud.