There isn't a way to add a gradient in the header with our standard OptimizePress options I'm afraid, only a solid colour or image are able to be used. So you could create a gradient image inside Photoshop which is the height of your header and then upload that as the tiled background
Alternatively, if you were not planning to have any menus as part of your header, you could consider using a normal row in the LiveEditor as a header - so you would hide the normal header (turn it off in the Layout Settings > Header & Navigation) and then add a row at the top of your page. You could then add your header image or logo there .
You would then need to open up the row options to customize the row top/bottom colour (to get the gradient effect). You may also want to change the padding for the row to fit your design.
This is a workaround but one you might consider if you dont want to have custom gradient image designed