On the Layout tab you will have a “Center Grid” checkbox available that makes sure the grid is in the center of the page:
But take note that this checkbox can only show up when “Layout Style” is set to “Masonry (Pinterest like)” and you don’t have a different layout style for tablet or mobile.
For most sites we recommend not using this “Center Grid” checkbox but rather having “Item Sizing” set to “Set number of Columns”, as in the screenshot above.
With that item sizing, the grid will always be full width and therefore look good (and “centered”) on any screen size. You can then set it to use a different number of columns on tablet and mobile.