How to create a gravity effect with Javascript?

Look a this jquery plugin on github JQuery.throwable just do $("Selector").throwable() and the object will be under gravity

You will want to start with a physics engine, the one Google Gravity uses is Box2Djs which is a javascript port of Box2D. You can read the manual for Box2D to learn how to use it, though the manual itself states that you will have little idea what you are doing without some knowledge of rigid body physics (force, impulse, torque, etc), though these examples may help you get started.

If you want to write the physics engine yourself you will have to at least implement 2D rigid body dynamics and collision detection for it to look like the examples you gave. A tutorial for doing that would be called a computer simulation class and would have a linear algebra and physics I prerequisite, it's not a trivial task.

Afterwards, you will have to learn about javascript animation techniques. I recommend learning about window.requestAnimationFrame. Using setInterval(stepFunction, time) will work but it won't be as efficient as it could be in modern browsers.