1) Consider sitting the free practice test in your account by clicking on Tests in the top Menu and scrolling down. One of the units with questions and explanations will be quite helpful for what you are looking for.
2) Dr Ferdinand does a one hour free online seminar every month (
http://www.gamsat-prep.com/news.php?newsid=9). Though physics will be next, biology questions from the blue booklet that you mentioned in your question, will be done in 2-3 months.
3) Our practice exams GS-1 to GS-5 and ACER's practice booklets also have further questions/answers to help.
Basically, you will improve once you complete many problems of this type and review the way the answer was determined.
