Gravity and movement in infinite platform

I am trying to make chrome dinosaur game . I have created a infinite platform with the help of this video :
so my character sticks to the ground and moves back. If I set the gravity to cant jump. Help ?

You’ll need to provide a screenshot of your events that jump and change gravity. We can’t tell what’s going on otherwise.

What to provide,I can’t understand

Gravity moves the character down the screen until they land on a platform. If the character is not on a platform, then they cannot jump because there is nothing for the character to jump against.