Additional Unown G's, Devoluters, and Skuntank G are universal techs that almost any deck can use against Gengar. Most can use Ditto too, as has been said. Most players don't expect Devoluters, and a KO with it may be all you need to get ahead for the game. I always try 2 Unown G and 2 Devoluters in a deck before considering anything else for Gengar match-ups.
Dusknoir Lv. X shouldn't be hard to play around. Just watch your bench for Dark Palms (Which is hard, but possible in a Flygon deck). Wait for them to Level Up, Warp Point them, and Extreme Attack for the KO and you avoid ectoplasm. Extreme Attack --> Psychic Restore/Poison Damage on Gengar Lv. X is another method to get around Fainting Spell (Just don't Warp Point, keep Gengar active to take advantage of its resistance and leave it at 130), and try to keep Nidoqueen off the field with Dark Palm. Extreme Attack is great with Skuntank G, you should drop Gengar Lv. X without a Fainting Spell every time.
If you get really desperate, and you really expect to play a ton of Gengars/Dusknoirs... consider a 1-1 Weavile SW (Keep it Guard'd, and Dark Engage + Psychic Restore is brutal) or 1-0-1 Ampharos PL (Just make sure you Guard up with Unown G before you play it) in addition to 3 Unown G's. But otherwise don't bother, Ampharos is sorta redundant in any other match-up because Gardevoir already stops powers and nothing else is really weak to Darkness.