In order to use Gson you have to include it in your project. You can do this by adding the following dependency of the Gson version available in Maven Central:
Maven
Add to pom.xml
<dependencies>
    <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.8.0</version>
      <scope>compile</scope>
    </dependency>
</dependencies>
Gradle:
Add to build.gradle
compile 'com.google.code.gson:gson:2.8.0'