I don't have enough information to say exactly who Gracie is. She could be an ordinary person whose story is being told because of the important choice she made.
In 'gracie's choice based on a true story', Gracie is the person at the heart of the narrative. We can assume she's an individual who has faced some sort of dilemma or situation that requires her to make a choice. She could be a young girl or a woman, and depending on the nature of the true story, she might be from a particular family, community, or background. Without more information, it's hard to say more about who she is exactly, but she's the one whose actions and decisions are the key elements of the story.
Since I don't have access to the actual 'burns and allen gracie tells a story', it's hard to say exactly what the story is. However, knowing Gracie from Burns and Allen, it might be a light - hearted story full of comical situations and witty remarks that were characteristic of their shows.