When he first came to her, Mary didn't believe Gabriel was real. She said as much. She said she believed in God and destiny and the salvation of mankind, but as far as angels go, she just wasn't convinced.
This was before Gabriel announced his revelation. He visited her a few times, just so she would become comfortable with him. In those days, angels were a little more active among humans, or at least more overt about it. Gabriel himself had visited many humans. He never really understood them, but at least he had experience with them. The first thing he didn't understand was how they saw him. Most of the reports he subsequently monitored mentioned wings. Gabriel had no wings. In fact, no angels had wings. True, he tended to float, because angels have no reference for hard surfaces, and so cannot simulate standing. They can simulate a lot of things, but not standing. Perhaps to rectify this aberration from everyday experience, humans simply rectified the difference, and assumed all that bright light was hiding wings, like birds.
The bright light. Well, it was another manifestation of angels not getting human conditions quite right. The sun doesn't exist where angels come from. Consequently it can sometimes be easy to assume that humans generated light. Gabriel was certainly aware of the Six Day Labors, and the myths some cultures had developed around humanity's acquisition of light, but it was all a little fuzzy. After all, it was all just theory to him.
Mary asked if he would sit down, actually. Gabriel had no idea how to respond to that. He didn't acknowledge it. He knew everything that was going to happen, omniscience borrowed from the big guy, as always, for visitations. He knew that Joseph was going to have his problems. He hoped to discuss this with Mary, but it was clear from the start that she wanted to make these decisions for herself, to believe that she still had an active role.
The only way he knew to convince her of his legitimacy was for Gabriel to show Mary what only he knew, which was what he did when he wasn't visiting humans. He brought her, just for a moment, to his observatory, where had access to all time. This was the first time in modern history that a human saw dinosaurs. That was all that Mary needed. It was seeing such awe-inspiring creatures that she understood both that Gabriel was what he said he was, but also that the world was everything that she had believed about it, far more massive in scope than most of the people she knew could ever comprehend.
In a strange sort of way, this makes a lot of sense. But it's not something you'll hear about too often.