Advanced Tables - classic Google Sites
How to insert a table into a Google Site and then remove the borders. This will require you to edit the HTML so if you don't feel you want to do that you will still have the borders.
Another approach to adding useful tables to your site is to use our Awesome Tables gadget that uses a Google Drive Spreadsheet for the content of the table and a gadget to format the table and provide filters and sorting. For more information see Google Sites - Awesome Table - Gadget.