Let's spend some time to check the Transparency Panel.
After we have applied the Opacity Mask in my last image the Transparency Panel looked like the one to the right:
We have 'expanded' the panel to include the 'extras' surrounded by yellow by clicking the 'up-down' arrow shown at the top-left of the palette.
First you notice that there are two thumbnails available (ITEMS 1 & 3), one showing the photograph (ITEM 1) and one showing the mask (ITEM 3) with a lock icon in between (ITEM 2). By default the photo and the mask are locked together and what this does is - let's say you decide to move the photo, then the mask is moved together. If for some reason you want to move the photograph but you do not want to move the mask (maybe you want to show another part of the photo), then click on the lock icon (it disappears...) and reposition the photo. The mask will stay in place.
If you now click on the Mask Thumbnail (ITEM 3), then you can edit the Gradient the way you would normally do. If you pick the Gradient Tool and 'drag' across the main image...(i.e. not within the Transparency palette, but on the main image on your artboard...)