This is the old way to install plugins. In older Gallery versions (2.0 and 2.1 (including 2.1.2)), installing modules and themes is a little more involved.
See Installing Additional Components for the new and easier way to install plugins via Downloadable Plugins.
You need to download the plugin from the official download page:
Most plugins come as .zip or .tar.gz file. First you need to extract them (e.g. with Winzip). Then you need to copy the plugin folder into the right folder of your Gallery 2 installation.
Example: We install the 'comment' module in this example:
We have downloaded g2-module-comment-1.1.1.zip After extracting the file, we have a modules/comment/ folder on our computer Now we need to copy the 'comment/' folder into the 'modules/' of our Gallery 2 installation on the server
Note: Themes must be copied into 'themes/' folder of your Gallery 2 installation.
Note: In case you installed Gallery 2 with the Pre-Installer, please see: How can I upload a plugin via FTP when I used the Pre-Installer?
Note: See the above section about Downloadable Plugins in case you're using a Gallery 2.2 or a newer version.