Your Guide to Using the Afterburner2 Template for Joomla
No, Afterburner2 is only compatible with Joomla 3.x. Joomla 1.5 support was discontinued in September 2012 by Joomla and 2.5 support ended at the end of 2014.
Gantry is, as a basic definition, a framework used for assembling, building and maintaining a RocketTheme template. It is an advanced platform for dramatically extending the capabilities of the entire theming system of Joomla.
DropDown Menu is a menu theme of the RokNavMenu extension, the addon that drives many RocketTheme template menus. It is primarily a CSS enhanced dropdown menu, with subtext line, multi-columns, icons and more.
No. RocketLauncher will install both Joomla and the demo sample content and images, so you will need a fresh Joomla! installation.