When you use one of the tiled gallery styles, like square tiles, it should automatically size and position the images so they take up the full width of the post or page. In other words, there's no need to worry about centering the gallery — it will take up all of the available space.
However, we've had reports that when you use a full-width page template, the gallery doesn't adjust properly. The images and layout are still being created as if you had a default-width page. As a result, the images are left-aligned instead of filling the entire width of the page. Our team is aware of the issue and is working on a fix.
At this point, there are some workarounds if you have the Custom Design upgrade and want to make changes to your theme's content width or make changes to the CSS. Otherwise, if you're concerned about the images being as large as possible and using the whole width of the content area, I'd recommend putting them in a post or using the default page template.
Please let me know if you have any questions about that!