Let's define the other screen sizes. Repeat the operation of selecting all your GridCol (Shift + click) and this time choose "Size 576-768 px (sm)" and define it at 6. It means that once the screen size is greater than 576px, the grid will be displayed as 6 columns on the left and 6 colums on the right. That's the basic principle of responsivness, how your grid will display depending on the space it has.