Create "Pages" page template options

My Google skills will simply not allow me to search on the right terms to find what I’m looking for, so I thought I’d try here.

One of the template pages I have setup in my theme is a form page. The page has content in the left column, controlled through the main content area, and the form itself (using Gravity Forms) hardcoded into the right column. I’d like to be able to remove this hardcoding option, and allow my client to setup multiple pages himself. Providing him with an option, when he chooses the “Form Page” template, to have an empty input box that he can drop in the shortcode of the form he wants to use.

I know I’ve seen this done on major theme providers templates, but have not found anything for being to accomplish it myself. Was hoping maybe someone knows a link to a tutorial about how I can accomplish this myself. Essentially providing more options for the user, when a particular “Page Template” is chosen when setting up the page.

Thanks in advance!