In WordPress Dashboard go to Appearance submenu and click on Customize.
After clicking on customize you will in theme Customizer.
Here you have to click on Page Template than Gallery Page
In Header Here you can manage Title & Description.
Go to Customize -> Page Template ->Gallery Page ->Header
In WordPress Dashboard go to the Appearance submenu and click on Customize.
After clicking on customize you will in theme Customizer.
Here you have to click on Page Template than Gallery Page
In Content Here you can manage Content.
Go to Customize -> Page Template ->Gallery Page ->Content
You have to use Gallery Image Size 510Px X 510Px