To include third party libraries you need to package your plugin in a folder before zipping it up into a gplugin. Any jar files that are in the root of that folder will automatically be included on the classpath. The folder name must match the name of your plugin class.
Are you using our plugin development kit from https://www.geneious.com/plugin-development ? The example build.xml files in there have a commented out section for folder type plugins.
Alternatively there are a number examples on https://github.com/biomatters. The biocode-lims is an example of a plugin that includes libraries using Apache Ant and Apache Ivy. If you're looking for a more modern build system then the geneious-eupathdb repository is built using Gradle.