Hi! I am new to this software and I would like to know how do you create a moving background as well as the platform? A good example are games like zombie tsunami.
I will be very grateful and appreciate every help on this. Thank you! 
Hi. You can move objects by using All Objects\Displacement events and you can also use the platformer extension to make a platformer game easily. I recommend to have a look the platformer and parallax scrolling examples included with GD.