If you want your editors to be able to easily apply your custom CSS classes via the Styles drop-down list in the GT Editor:
Are you adding design elements with CSS and want editors to be able to use your new CSS classes within GT Editor? You may have discovered that those classes are being stripped out when the page is saved. What's happening is that GT Editor operates on a whitelist principle to keep your site and its content safe. You must add any elements, classes, etc.
A "rich text" or "WYSIWYG" (What You See is What You Get) text editor gives your content editors a way to create and maintain content using an interface that looks and feels a lot like using a word processor. Formatting is applied using button controls, so the content editor doesn't have to know anything about HTML and can immediately see what the content is going to look like after it has been saved.