I couldn’t make google gears work with the new Safari 4 Beta, until now..!!
If you have the same problem, read on….
First open a terminal and uninstall any previous gears by running this command
/Library/Internet Plug-Ins/Gears.plugin/Contents/Resources/uninstall.command
After uninstalling, open Safari 4 and go to the install page and re-install the latest version for Safari 4 Beta.