Tom,
I have not dealt with gnats in my orchids, so if they are in bark mix (or similar) this will not work, but if the gnats are in more "traditional" potting mix, I have an easy solution. It worked wonders for my potted citrus.
Simply take normal sand (I use All-Purpose because I have it around) and work it into the first 2-3in, so there is a noticeable difference in the soil consistency. Then, and here is the trick, pour another 1-2in of pure sand on top of the existing soil. Do not work it in to the rest of the soil. I just filled it to the brim (since I generally leave about an inch between the soil and pot for ease of watering). I found that the larvae can only live in the first inch or two and that their food sources do not exist in pure sand. They starve to death.
After a few months, I take the sand off and it goes in the compost, but the gnat life cycle is significantly disrupted. The only downside to this approach is that it forces you to slow down your watering or you will overflow the pot, which makes an awful mess with all the sand.
I do not know if anyone else has done this because I thought of it on my own (although I am sure others must have too), but it worked for me. I hope it helps.
Best,
Per