Hey
The Gradmed questions are set by design to be harder than the actual exam, but common feeling is that Medprep is too hard... go figure

. People recommend different courses depending on background (arts/science grad?), place in country, personal preference...
There is no official syllabus for the GAMSAT thus you will have to buy something: all they say is that it tests "first year uni biology and chemistry and end of school physics". The notes of course evolve year on year but they're made by people who only find out what's in the exam by going and sitting it themselves, so ultimately I doubt they change all that much!
I'm using the Des notes from last year (cheap) and some textbooks. I really recommend the Palgave Foundations books in Biology and Chemistry if you can get them... they're for people who're starting from scratch going to first year uni level, which is Gamsat, and Organic Chemistry for Dummies is also a huge favourite here

. You will also need the prep materials from UCAS - Sample Questions and a Practice Test.
Basically, I'm hoping to survive on all of that and it wasn't too expensive, but in this game you can spend anything up to thousands of pounds. The key is just to get started ASAP.