Is atheism about devotion and loyalty or factuality?
In the broadest sense, atheism is simply the absence of belief in the existence of deities, it is not a positive assertion.
Agnosticism implies you do not know one way or another...
Right, it's the absence of knowledge, or the view that the truth is unknown and/or unknowable.
If you were roleplaying in a Stargate setting based on the original movie, I think all of the following beliefs would be valid character choices:
Gnostic theist: I believe that Ra is a god, I've seen the miracles he performs.
Gnostic atheist: I don't believe that Ra is a god, the evidence proves that he's just a Goa'uld.
Agnostic theist: I believe that Ra is a god, but I don't think there's enough evidence to prove it.
Agnostic atheist: I don't believe that Ra is a god, but the evidence doesn't prove he isn't one.
Of course all four viewpoints recognise that there's a guy called Ra, and I doubt anyone would dispute that he's powerful, but not everyone would consider him a god. I think the same could apply to many fantasy settings.