Hi there,
your site is loading boostrap.css which is adding this CSS rule:
.container, .container-fluid, .container-lg, .container-md, .container-sm, .container-xl {
width: 100%;
padding-right: 15px;
padding-left: 15px;
margin-right: auto;
margin-left:auto
}
Which effects the GP .container element ie. adding 15px to either size.
Ideally you would want to remove bootstrap from the equation ( it loads a LOT of CSS and will have several conflicts ).
If you cannot then you can add this CSS to remove the padding:
.site.container {
padding: 0;
}