When to sit the GAMSAT- September or March?Watch
September is good as you will have the summer to prepare without having to do as much GAMSAT prep alongside your uni work. But it means you will be going into your application blind (though you will somewhat if you sit in March - although a score of say 70 overall is really safe for getting an interview).
Both cycles last for the following 2 years. So March and September 2018 sittings are valid for 2019 and 2020 entry.
Hope this helps!