you have 40px left and right padding on the Container and 40px padding on the Grid columns.
Try removing the padding on either the container or the grid item form mobile devices.
You can find them several ways using the Developers tools ( Right Click > Inspect ).
The way i found this was by inspecting each GB Container and deleting them one at a time from the elements list in the dev tools. Once the overflow corrected itself i knew the problem was in that container. Then i refreshed and inspected the inner elements of that container – the extra padding was easily visible.