I'm self taught as well, so there may be an easier way of doing this.
But this is how I would do it!
If you have drawn everything on a layer above the 'Background' layer, it should be simple enough to simply stick your gradient up on the Background.
However, if your picture is drawn on, or is set as, the background layer, you will have to copy-paste the picture onto a New Layer. Make it not visible while you switch to the background layer, then make your gradient. Now make your New Layer visible. Then you can erase the redundant background from this new layer and viola, simple gradient backgroundy dealy.