If they were on internal storage (there's no "gallery section" - Gallery displays whatever is on th phone, internal and external), you can install the paid version of DiskDigger. (You may overwrite one or more deleted files when you do.) It should recover any files that weren't overwritten since you deleted them.
Of you can look at
munky-tool and do the same things through adb without risking overwriting anything. (Don't create the image file on internal storage - create it on the SD card. A discussion about the script is at
munky-tool thread.
If they were on the external card, just put the card into a card reader, plug it into a PC (Linux, MacOS or Windows) and run
PhotoRec on it. If there's a file on the card, PhotoRec will recover it, whether it's deleted or not. (Unless you tell it to recover files only from unused space which, while saving time, may miss some files.)
If you use munky-tool, you'll have an image of the data partition of the phone, and you can use PhotoRec on that image to recover files. (Read the iPhone section at the site. The way of creating the image is different, but once you have the image file the recovery is the same.)