managing content on existing website

[Timbow]

You will need to rebuild it, but not rewrite it. If your html is reasonably tidy then it is not a big task.

You would make your site layout into a custom GS Theme by taking the main html page, renaming it template.php and then by referring to an existing theme template like Cardinal you would replace bits of html with the php tags.
If you have different page layouts for different pages you will need a different php template for each one.