<p>I've been hearing conflicting facts about this topic. What is correct?</p>

You've been hearing conflicting views because it's a topic of great confusion, even among senior engineers. In short, simply placing an assembly in the GAC does implicitly give it full trust, but this can be overriden via security policy.
EDIT1: Let me add that a common thought is if you don't trust an assembly fully, why are you placing it in the GAC?
EDIT2: I had a link in here to a blog post from Michelle Bustamante, but as you can see in the comment below, it's no longer available so I removed it from this answer.