I imagine such a creature would be composed of thousands, possibly millions of planets, gas clouds, asteroids and stars. Almost like a giant living engine powered entirely by gravity.
Obviously the time scale this creature would live and perceive on would be vastly slower than ours. Taking information over such incredible distances would take millions of years.
Would we even be able to detect such a creature? I imagine we would simply see it as planets and stars following their normal path. I guess humanity would blink in and out of existence in the time it took the creature to think a single thought. We wouldn't even know we were living inside it like a form of bacteria. Our satellites, radio signals and space stations might even be harmful to the overall working of such a being.
It's biology would be like nothing we have ever seen. I'm happy to leave that up to the readers to figure out. Whether it passes information / signals around it's "body" via gravitational fields or asteroids etc.
Are there any killer physics principles that would absolutely rule out the existance of such a creature?