Hey SkiingPilot,
nice to hear you got invited for the GU (Basic Testing).
This is the well known DLR (German Psychometric Test on the highest possible standard). If you read this entire Thread except for the whining part on the first couple of pages (like every PPRuNe thread), you'll get a good impression.
From my point of view there is nothing more to learn in this stage.
Also via your career.aero login -> downloads, you should find enough info regarding all the stages.
Good luck on your Prep!