This page contains a bookmarklet that makes it easy to create Gears manifest files without having to write them by hand. It will automatically identify all the resources you want to make available offline, such as images, stylesheets, etc. and then generate a manifest file for you.
To use, drag the following bookmarklet to your links toolbar. Then, navigate to the web page that you would like to offline enable. Once pressed, this bookmarklet will scan the page, identify all the resources that you might want to have offline, and automatically generate a Gears offline manifest file for you. This manifest file will appear in a text area on your screen -- you can now edit this file, then copy and save it into a new file, such as manifest.js.
Note that this script only works on Firefox and Safari due to limitations in Internet Explorer on the length of bookmarklets.