Which major - and where at - would best provide ample preparation for the GAMSAT? i studied year 11/12 chem and physics but not biology and thats the worry.
'Quietplease' please do not forget to consider that half your exam will be on the humanities side/critical reasoning side. If possible and if you like humanities consider doing some as electives.
But yeh most Gamsat applicants are from a science background as long as you cover yr 12 physics, organic chem/1st year chem and 1st year bio as Tim035 stated you should be fine.