Ah, Gaath doesn't actually melt people completely into puddles; that particular ability works by having its wielder being capable of exuding a very potent solvent that is mainly effective against skin, hair and flesh, but not very effective on bone. Not only would the dissolving of an entire person probably take him some thirty minutes, making it much too dangerous to stick around that long in most cases, but the skeleton (and various inorganic possessions) would remain mostly intact. Interestingly it is less frequently the melting itself that kills with this ability, but rather the noxious fumes resulting from the solvent reacting to the victim's skin and flesh that ends up choking the person to death or poisoning them... that, or an artery or air-pipe being dissolved.
I don't think Ixion should have actually seen Gaath, though... both because Gaath was pretty notable to look at even if he's not doing anything a month ago - his immortal blood being quite evident from his appearance - and since Ixion would probably have witnessed Gaath using some of his other abilities too, then. And of course the fact that Gaath didn't usually go near larger populated areas like the cities due to his aforementioned unusual appearance.

But a month ago, yeah... around that time he should have been in southwestern Gilmah, I think. Four months ago he did have his melting-ability, but if Ixion heard of him from back then, he would have been in... in northern Wenal, actually, near the Savage Woods.