Plugin template files can be found in the /wp-content/plugins/gallery-plugin/templates/ directory.
You can edit these files in an upgrade-safe way using overrides. Copy them into a directory with your theme named /bws-templates. Example: To override the single gallery template, please copy /wp-content/plugins/gallery-plugin/templates/gallery-single-template.php to wp-content/themes/your-theme/bws-templates/gallery-single-template.php.
Do not edit these files in the plugin's core directly as they are overwritten during the upgrade process and any customizations will be lost.