How do I apply gravity after ball bounces off?

You need to set gravity after spawning, not before.

So you need the ball to bounce up and down? Then just the gravity should be enough, no need to set the bullet speed or angle of motion. The ball will fall down, bounce off solid floor, go up, then fall down again.

This was what I did at first but issue is when I remove the bullet angle of motion, the balls moves from left to right and not downwards

So i changed the angle of the invisible spawning object to 90 so it forces the ball to go downwards but when it collides with the paddle, it move upwards and never comes down

You are doing something wrong.

Create a new sprite, add Bullet behavior, set speed=0, gravity=100, "Set angle=no"

Run in preview and you will see that the sprite starts to fall down.

If you set "Bounce off solids=Yes" and add a solid floor below, the sprite will start bouncing up and down.