I wouldn't use tables but rather Gantry reponsive grids
http://docs.gantry.org/gantry5/advanced/responsive-content
.
But you can make your table scroll like this:
table {
max-width: 100% !important;;
overflow-x: auto;
display: block;
width: auto !important;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.