There are plently of ways to revise for a german reading exam, all of which will make a difference to your grade.
Firstly, it is really good to get into the habit of reading German news articles, one because this will help you effectively prepare for the reading exam and two because it will broaden your knowledge of German to a very high degree. If you are familiar with German articles then the articles or short texts presented to you in the reading exam will not come as a surprise. The best website to find German news articles which are not of standard too difficult is www.deutschewelle.de . Deutsche Welle is also available in English so the great thing is, once you have read an article, and only after you have read an article, you can check how well you understood it.
This takes us on to the second way to revise - build vocabulary bank and learn the words which you note down. When learning a noun, learn the singular and plural forms as some plurals in German add umlauts to the singular forms which may cause you to not recognise a noun you actually know in the exam. There are no excuses to not having vocab to put in your vocabularly bank because the first way of revising for a reading exam will provide you with plently of new material.
Thirdy, do some past papers and if you unsure where to find these then contact me telling me which exam board you are taking you German GCSE with and I can provide you with some links. By doing past papers you can familiarise yourself with the style of the exam you will sit, for example which sections are mutiple choice, which sections you must write in English and which ones you must write in German. It is also a great skill for practicing your time management and is reassuring when you know from your practice that you will finish the exam.
If there are any particular practice questions or parts of a GCSE German reading exam which you cannot get your head around, then book a session with me to solve any issues or worries you have.
Good luck revising!