As a plugin developer you can use this plugin as dependency of your plugin by adding a dependency tag to your POM.
To add the latest version of this plugin as a maven dependency, use the following:
<dependency>
<groupId>org.jenkins-ci.plugins</groupId>
<artifactId>gravatar</artifactId>
<version>213.212.v09426a_2eb_553</version>
</dependency>