Where exactly are you trying to use the RapidGallery content plugin? Joomla content plugins are intended for standard Joomla content (i.e. as Articles). If you're trying to use the plugin into a custom Module and/custom component or plain text areas which don't trigger Joomla plugins then it's normal seeing the uninterpreted plugin code syntax; as you're using it in the wrong place.
Here are some frequent misconceptions: Joomla 1.5 custom HTML module does NOT trigger ANY Joomla content plugin. Virtuemart product SHORT description field is just a plain text field and it does NOT trigger ANY Joomla content plugin.