Neither of those skins look even remotely 40K inspired. Dark Star Cho'Gath doesn't even look close to how Tyranids look. Tyranids are organic, Dark Star Cho'Gath is artificial-looking, spacey, but still physical.
God-King Garen, similarly, looks near-nothing like Guilliman or the God-Emperor of Mankind, outside of the fact that he has big, bulky armor and a divine countenance. God-King Garen is really more leaning towards fantasy paladin designs than 40K Space Marines.
If you really want an example of a skin with influences from Warhammer 40K, look no further than Steel Legion Garen. He's got bulky, futuristic armor, and he's got a friggin' powersword.