You would probably be better and easier just to add a new gallery and start again, unless you want to go through and add all your items through code in a items array if you wanted to stick with the Wix Pro Gallery app option.
https://www.wix.com/corvid/reference/$w.Gallery.html#items
Set the list of items for a gallery
$w("#myGallery").items = [{
"type": "image",
"alt": "A beautiful view",
"title": "A View",
"src": "wix:image://v1/99bc1c6f66444769b531221214c885ac.jpeg/A%20View.jpeg#originWidth=3264&originHeight=2448"
}, {
"type": "video",
"description": "Another beautiful view",
"title": "Another View",
"src": "wix:video://v1/80c05f_e2c524db1f264178a8558c92dbf76fb0/_#posterUri=80c05f_e2c524db1f264178a8558c92dbf76fb0f000.jpg&posterWidth=1920&posterHeight=1080"
}];
https://www.wix.com/corvid/reference/$w.Gallery.html
https://www.wix.com/corvid/reference/$w.GalleryItemChangedEvent.html
https://www.wix.com/corvid/reference/$w.GalleryItemClickedEvent.html