Gradle is not an IntelliJ plugin.
Gradle is a program that handles compiling your code. IntelliJ includes a plugin that provides integration with the real Gradle.
As for updating, Battlecode uses a feature of Gradle known as the wrapper. In a nutshell, this means that the project automatically downloads Gradle for you. It is currently set to 3.3 already; you can check
gradle/wrapper/gradle-wrapper.properties to see and update the Gradle version.