There are some sample questions on the education portal.
These really helped me as well as the learning assessments on the GRC300 course.
That's what I use to train people who are sitting the exam.
The certification is focussed on GRC 10. I am an instructor in the UK for the GRC300 course so its difficult to say how much prep I did. I actually did no specific prep for the exam, just turned up and took it but I have been working with it for a while now so its not exactly a fair comparison.
If you are definitely looking at taking the exam, I would highly recommend that you attend the GRC300 course first, especially if you have no exposure to GRC 10 at present. You'll really struggle to pass the exam without having access to the course content.
This is the course for GRC 5.3
There is not a certification for AC 5.3 only for AC 10.0
If you look at the certification section of the SAP Education Website, you can see sample questions, and where the questions are being taken from.
Sr Consultant - Access Control
Business Analytics Competency Center