I downloaded the Gson library from http://code.google.com/p/google-gson/. The archive contains the following jar files:
google-gson-1.3/gson-1.3-javadoc.jar
goog
Right Click on the project, Build Path->Add External Archives
Here is the youtube link which answers this question. it shows how to step by step add gson to Java. https://www.youtube.com/watch?v=fzdIcbnnwSA
1)Add google-gson-1.3/gson-1.3.jar as external library
2) click on the new added library and add google-gson-1.3/gson-1.3-sources.jar to source attachment
3) click on the new added library and add javadoc google-gson-1.3/gson-1.3-javadoc.jar to javadoc location.