I'm tossing up between Micorobiology/Genetics.
The only reason i went with genetics is because Microbiology apparently requires some maths skills.
http://bsc.unimelb.edu.au/microbiology-infection-immunologyThe sample course plan has Calculus 2 and Linear Algebrah to complement the major.
Thought you should know

Maybe someone doing the major can shed some light on how maths intensive it is.
Also as Peedles said
Personally, I don't think that you should choose subjects that you think will benefit you in gamsat at the expense of you not enjoying it and doing badly and it having an impact on your GPA.
Do the major you enjoy; a genetics major certainly won't disadvantage you in the GAMSAT!!
GAMSAT section 3 is such a broad section and only tests upto 1st year uni knowledge so i don't think micro over genetics will give you an advantage. GAMSAT will never ask you questions specifically obtained from 3rd year microbiology or genetics material.
Not to mention both majors share some subjects together so they are somewhat related.
P.S. On a personal note, I hope you do Genetics so you can help me out during exam times again
