Theres a trick to them. As with every unit, you just need to find the right counter. In the case of Gaesatae, the counter is slingers. Get a few units of decent slingers (a few chevrons helps massively) and target the nudists, preferably from their right or their behind, and you will wreak carnage upon them... I imagine that no matter what the drugs you're on, a stone travelling at over 100 mph on bare flesh (not to mention if the slinger is really good, and can cherry-pick his target, so to speak) is going to hurt...

Failing that, heavy cavalry are still pretty effective against them, on account of not getting scared by them. Just dont get bogged down in melee with them, and they are not too bad.