Hi Prat,
for some reason you do not have connectivity to eclipse.org repository (that provide gson library).
Do the following:
1. Ensure that you have correct proxy set in Windows/Preferences/General/Network Connections
2. Reload all update site repositories that are available in your Eclipse - go to Windows/Preferences/Install Update/Available Software Sites
- Select sites one by one and click on the "Reload" button.
3. Try the installation again.
If this does not help - you may want to try clear installation by downloading the Eclipse Kepler for Java EE developers from here:
https://www.eclipse.org/downloads/packages/eclipse-ide-java-ee-developers/keplersr2
Best,
Dobri