I assume those intending to sit for GAMSAT will go for the one in Sep? Because the upcoming one is in March and it will be a mad scramble to have everything down pat in 2 months (us people who have little to no science background definitely need more time than that!)
I have spent countless hours trawling forums to come up with a good study plan and have been at it one-off for the past 6 months (I work full-time and don't get much time in the evenings). Ok, so here it goes:
Section 1:
Get MCAT resources (ExamKracker or The Princeton Hyperlearning) to get used to long, convoluted passages. Its very important to be able to digest such passages and this is achieved only through practice. Des O'Neill has a humanities workbook for Gamsat, which I have not started on yet, but I'm sure it will be helpful.
Do a few passages daily if possible and if you think it will be a piece of cake given our background, think twice (or thrice). I have read a good number of classics and count myself as well read but the passages (and the darn questions!) have a way of thoroughly confusing you. So please start early to know where you stand.
Section 3:
Get the 'Griffiths gamsat review' pdf, if possible, because it lists down core topics to study. There is no one perfect text book or resource to understand the science material and you need to try out the suggestions from people in TSR or pagingdr forum (wealth of info for us). After trying out a few resources, I have settle for these:
Org chem: Organic Chemistry as a Second Language (first semester) - I understood this book much much better than the Organic Chemistry for Dummies. There are multiple practice questions (non-gamsat style) throughout the book to test your understanding.
General chem, Bio, Biochem: Examkrackers - this is a MCAT resource with good illustrations and concise explanations. I also googled any niggling (sometimes elementary) questions when the books weren't too detailed. Knowing that I did not have tonnes of chapters to cover helped psychologically.
Learning content is just half the battle. Practicing Gamsat style questions is what we need and you can count on Des Science mcq book for hundreds of passages.
Haven't had much time to dedicate time to physics and section 2, so I have nothing to add here.
So in summary, get griffiths Gamsat review, Des O'Neill Mcq pdf/books (esp science) and whichever textbooks that work for you.
Cheers