Java Pong implementation
Thanks for sharing your code.

Naming

Finding good names is the hardest part in programming, so always take your time to think about the names of your identifiers.

Naming Conventions

Please read and follow the Java Naming Conventions 

eg.:  


Let class names always start with an uppercase letter
let boolean variables and methods returning a boolean value start with is, has, can or alike. (isGameRunning vs. gameOn)


Choose you names from the problem domain, not from the technical solution.

eg.: your class keyListener might better be named PadelControl

don't use comments to "save" code

Modern IDEs keep a history of the last saved versions of your code files. They have capability to compare with this old versions and get them back (or parts of). Also using a source code management system like git or svn is quite easy for the same purpose. 

So there is no need to keep "inactive code" as comments in your code files. just remove them.

use adapter classes if possible

Almost all Swing interfaces have empty implementation classes called *Adapter in your case it is the KeyAdapter. You can extend that class and only implement the method you need, keeping your code clean from the empty implementation of the unneeded methods.

do not add code with unknown/unverified behavior

You method go() ends with an empty loop. 
I suspect you added this because you thought you program would exit otherwise. Maybe you have been told to do so or red somewhere.
But you most likely never tested that behavior.

This empty loop results in 100% activity of one of your CPUs cores. Some years ago, when there was only one core your computer would have been unusable after the start of your program. Thanks to technology advance this loop does not stall your PC.

So never add code just because it sounds like a good idea, always test that certain code really has the expected effect. If it doesn't delete it immediately.


  There is a programing technique called Test Driven Development supporting that.