sorry, i'll try make a bit more sence

I want to make an image like the one (at happy cog) which creates the big shadow effect all down the page. I understand that this is some sort of gradient effect but unsure of how you go about making an image with the gradient effect on the left and right hand side of the image. (not sure if that makes sence )

could you tell me how i go about creating the shadow image that is used for happy cog?

sorry if i'm still sounding dumb , cheers 4 the help tho