Hi Roger,
As the gallery is the custom post type that we have used before in the theme. There are lots of users who uses gallery. So we cannot change that name which is hard coded in custom post type. If you use permalink than your gallery link will be displayed as domainname/?post_type=gallery&p=626
If you want to change it then it needs code customization and you need to hire a developer to fix it. Create a child theme before making any changes on it.
Also you can use WordPress default gallery if you don’t want to use it.
Thank you!