Well, he never meant to become what he is, and he's not specifically a lich- that's just the best comparison I could come up with for what he is. Gwyn (the character) has used a combination of his half-finished Soma/Elixir of Life recipe, plus magical technology, to preserve his life for so long. He's basically a steampunk or Jules Verne-style cyborg or other magical construct. He can live indefinitely for as long as he keeps replacing parts and maintaining his bodily integrity, but he can still be killed if someone stabs him enough times or destroys his physical body. He's not fully immortal- just extraordinarily well-preserved. Think the Frankenstein monster, if instead Frankenstein inflicted his treatment on himself.
Gwyn does know necromancy, though he usually uses it purely for communication purposes. Some of the methods and rituals Gwyn's used in his designs do stray uncomfortably close to the darker parts of necromancy, though, as you've mentioned, which is what I thought made the character interesting- Gwyn is a serious gray area to the magical community at large. He's an artificially undead abomination against nature with molten iron and eldritch substances flowing through his veins instead of blood, but he's also a fundamentally good person who uses his considerable talents and experiences to help people (even if nowadays he does it for mostly selfish purposes, such as to stave off his boredom).