Back when I sat Gamsat in 2005 there were only 2 other prep companies in existence. Today there are many more but nearly all of them still suffer from the same problems.
They're all basically only selling two things - a science textbook with a Gamsat label and a question bank tacked on.
Virtually no real analysis of the test.
Worse than this, because the science isn't focused on Gamsat in any way, many students are wasting valuable hours studying far too much detailed information which is very unlikely to come up in the test.
The second problem is the question banks...
1.Low Quality Questions - huge question banks of low quality questions which don't replicate genuine Gamsat style questions.
2. Wrong Question Styles - questions testing memorised information which rarely happens in Gamsat or questions which are not at the right level of difficulty.
3. Insufficient solutions - questions with poorly detailed or no solutions which don't help you learn.
4. Failure to include the official questions in your preparation plan - ACER publish 5 question books of their own but I've yet to see anyone else telling you the best way to incorporate these into your studies and when.