A rigid body will fall by default as soon as you add it. I'm assuming you're in 2D, since you mention the default gravity value. Add a rigid body, attach a sprite for appearance and a CollisionShape2D for shape and you should be good.
Once you have the body, adjusting bounce, mass, etc. is done via the body's properties, in particular the physics_material.
You might find the following links helpful in learning how to use rigid bodies:
Godot Docs: Physics introduction: RigidBody2D
KidsCanCode: Using Rigid Bodies