How would natural cycles (day/night, moon phases) change if gravity did not
exist?

Eh, we would be dead...

Explanation: First off, the moon is orbiting us for one reason, gravity. and if the switch for gravity just flicked off, poof! The moon would be speeding away at 1,000-2,000 hours! And the reason that we orbit the sun is gravity too. So every planet would fling into space. Everything in the UNIVERSE would fling into space. That much flying debris would eventually smash into earth. Well, if we did survive that, then we would freeze instantly because when we travel too far away from the star that we orbit, the planet would become very life-threateningly cold. And in this case, we are traveling away from the sun, which is the star we orbit. If we still manage to survive THAT, then we...would actually die from the start, cuz what holds our planet together? You guessed it-gravity.