The science section GAMSAT that I took was massively organic chemistry, no physical chemistry, some biology and some very simple physics.
The physics section I could have done with no previous physics training, the physical chemistry (had it come up) i probably would have bombed, I'm a biology grad and "organic chem for dummies" is an ingenious conception penned by Zeus Himself (that saw me thorough the organic chem)
The GURU only served to waste my time. It was not the only material that I used and was by far the worst.
I certainly wouldn't suggest you buy it (There was someone giving it away free the other day - that's all it's worth).
I suggest:
Organic chem I for dummies (naturally)
Letts A level physics / bio /chem books
Griffith's Gamsat Review
Maybe a 1st year uni biology book (Life perhaps, but this is probably going too far)
Practice papers
Des O'Neil is worth getting if you can get a copy for a few quid, but is very dry and not very plainly set out.