This was prompted by people talking on tumblr about whether or not Gates knew about Caskett. I wrote it before Still aired.
She watches him. She sees the way he looks at her. He doesn't know that she sees it, but she does. She sees the way his eyes light up when she enters the room. The way he makes sure she has her coffee. The way he ebbs and flows with her, in synch as if they were physically connected. The way his smile for her crinkles his eyes differently than his smile for anyone else does. She sees the way his emotions are a reflection of hers.
She also is well aware of how much he affects her. How he brings joy into her life, a kind of joy she's never known before. How she is a better detective with him around, even if it is only because there is less worry and strife in her life. He calms her, settles her, and uplifts her. He loves her, all of her, for who she is and was and will be.
She knows they complete each other. She doesn't care how cheesy it sounds. They are two sides of a coin, opposites attracting each other in a fierce and almost inexplicable way. She often thinks on how surprising it is that it took them so long to get where they are.
Yes, Gates knows that Beckett and Castle are together. She's known since the day Beckett quit. Because one of the only things Beckett loves more than her job is that man. And that love, as well as Castle's love for Beckett, had begun long before that day. That was evident from his books.
Gates knows that they think she doesn't know. And for now, she's going to let them keep thinking that. Because it's rather fun watching them try to keep it a secret. But a love that strong can't stay hidden. That's how she knows.