The best GAMSAT resources begin with ACER, the official source of the exam itself. Currently, they offer 5 eBooks (pdf files watermarked with personal information), and an automated essay-correction service ("Online Test Generator"). Some students feel that the eBooks have outdated questions, but most students feel that completing ACER's eBooks, with a careful analysis of the worked solutions, can be a helpful part of a successful GAMSAT preparation. The Online Test Generator is a reasonable predictor of Section 2 scores for most students.
Aside from ACER, to round off the top 3 best GAMSAT resources, and certainly the most-used resources over the years, would be Des O'neill and a company that you may have heard about: Gold Standard GAMSAT! From 2005 to 2014, Des was the best in terms of the similarity of their Section 3 practice questions with the real exam. In 2014, Des O'neill ceased operations and the relevance of their practice questions which, for example, were initially constructed when calculators were permitted, waned. Obviously, this is not the same as saying "irrelevant" or "no longer helpful."
Gold Standard's resources are far more extensive (7 books, 300 videos, 15 full-length mock exams), varied (paperback, pdf, apps and other multimedia) and up-to-date as compared to ACER and Des. Naturally, the best GAMSAT resources are primarily dependent on your academic background and the way that you learn best. If you have the time, try different materials to find the fit that is best for you. You may also consider browsing some student opinions, for example: Gold Standard GAMSAT Reviews.
To help you prepare for the exam, Gold Standard GAMSAT has assembled comprehensive information on GAMSAT scores, topics covered, preparation advice, free GAMSAT sample questions and other study resources.