03-css-specify-a-grid-gutter-size-with-grid-gap

Grid-gap and fr unit

        .container {
            display: grid;
            height: 100vh;
            grid-gap: 10px 20px;
            /*
                grid-row-gap: 10px;
                grid-column-gap: 20px;
            */
            grid-template-columns: 1fr 2fr 1fr;
        }

DEMO