It's a really old model ported from Dark Crusade back in like 2006 (yes they have that many polygons I think but the default textures suck ass) and has been on garrysmod.org for years, along with a pack of other EXTREMELY hastily and half-arsedly ported DoW 1 models.
There's one big issue with the version found on garrysmod.org though. Due to a physics problem (which I fixed in this version), it spazzes out and launches into orbit the second you click the spawn icon. I also added some detail to the texture & added normal maps, self-illumination and phong shaders.
The gauss flayer was ported from Fallout 3 by Omolong after which I converted the textures for use in Source.
I wish I had more Necron models. Immortals and Pariahs, Scarabs, Destroyers and maybe Tomb Spyders would be boss.
I know of the model pack you're speaking of, I downloaded it once so I could use the Basilisk alongside Bloo's IG models and the DOW2 models that Necrobard ported, it was also horribly ported. I wish somebody would port and improve the models, because DoWI has alot more models that DoWII