While I am not SC or even long with the site, my reading of the rules of basic editing says all edits must be to the entire image except dust and scratches filter and USM. Gradient by definition applies an effect more to one area than another so I belive it would be illegal in basic editing. As for member challenges with advanced editing rules I'm not sure but I think It would be legal.
While we are at it, have been going over the Advanced Editing Rules (I'm a noob, just joined today)...Is gaussian blur allowed in advanced editing challenges? Can one use a mask to blur just certain areas? Thanks in advance.