There is no way to only delete photos on Gather, but you can change your photos at any time.

To edit your photos:

1. Within the Gather app, tap your Photo Icon on the far right of the navigation bar
2. Tap the X on a photo to replace it.
3. Select the photo you want, click Choose, then move & scale your photo. You can even add and edit captions!
4. Hit Done at the top right when you're happy with the changes