You mean, both the HelloWorld class from the lwjgl.org/guide site as well as your GameClient class are in the same "project" and just in different packages?
And when you have the main() method in HelloWorld it works, but when you have it in the GameClient it doesn't?
This sounds completely illogical.
Question: If you use an IDE where you can configure the classpath, librarypath and JVM arguments in a "run configuration" per class, did you probably configure two different run configurations for both the HelloWorld and the GameClient class when you run them?
Please show the command line arguments with which both classes are being run.