Hi there!
I'm not familiar with HP Gallery, is it maybe called HP Photosmart? Are you unable to delete a photo and are trying to figure out how to do so, or do you know how to delete a photo and are trying to figure out how to delete multiple at once.
If it's the former question and you're trying to delete a photo, try going to the pictures folder on your computer and looking for an HP folder where you can directly delete the images instead of going the HP Gallery tool.
If it's the second question, try selecting multiple images at once by holding the ctrl button while clicking on each picture you want to delete. I'm not familiar with HP Gallery but this may allow you to select multiple pictures at once and delete them as a batch.
Hope this helps!
I am an HP employee