NoBots will block web crawlers from indexing your Gallery.
NoBots is a module for Gallery 3 which will add a few HTML meta tags to the top of your album and photo pages that will tell search engines such as Google and Yahoo to go away. This plugin is useful if you don't want people to find your gallery through search engines.
This module may be downloaded from the Gallery forums here.
To install, extract the "nobots" folder from the zip file into your Gallery 3 modules folder. Afterwards log into your Gallery web site as an administrator and activate the module in the Admin -> Modules menu.