How to retrieve deleted Photos from my iPhone gallery?

Deleted pictures can be seen in the Recently Deleted view in the Utilities section for 30 days, and they can be restored from there. If you choose to bypass this safety feature and delete them from the Recently Deleted view, then they are gone.

You can see this:

         If you're missing photos or videos in the Photos app - Apple Support

Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location.

If you use iCloud Photos,  you can look at Photos in iCloud.com. But they've probably already been deleted there, unless you're very fast.

If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data from somewhere within the last 30 days or so at iCloud.com, but you would need to look at the date of the backup to be sure it's from before you lost the images. You should look here:

         View and manage iCloud device backups - Apple Support

This requires you to erase everything on your phone and replace it with the backup.

If you don't keep your own backups to an external drive, then there really isn't any place else to look.