The goal here is to get a full-width page with no left or right modules showing.

First, you will need to create a menu item linking to the page to be displayed. This can be anything you want such as a gallery, wrapper, forum etc.

Next, go to the Module Manager (Modules > Site Modules).
The filter drop down is your best friend within the Module Manager. To the right you will see a "select position" drop down list.
Select the LEFT position.

A list of all modules available in the LEFT position are now displayed. Obviously the only ones of importance are the ones that are published. Starting at the top, click on a (published) module title link.

When the page loads you will see several parameters. Look to the ones at the right of the page labeled "Pages/Items". These are the menu items (or pages) which the module is to be present in its respected position, in this case LEFT. Use your control or cmd (mac) or cntrl (pc) key to select or deselect multiple items. You can also use the shift key to select multiple items in a row. The menu item you created to be full width should NOT be selected because you don't want this or any other module in the left column.

Make your way down the list in your Module Manager through all the LEFT positioned modules with the above method and do the same for the RIGHT position.