September Vs March Gamsat
No the September Gamsat is not easier than the March Gamsat. The test administrator ACER sets the degree of difficulty to be same in both sittings and then statistically adjusts results to make sure both sittings are directly comparable. This is why your result is valid for two years.
The idea that September might be easier arose from speculation on student forums that because less people sit the test in September there is less competition and hence average scores could be higher or you might place in a higher percentile.
However as mentioned above this is not the case because ACER employs a statistical process to make sure that results from different sittings are directly equivalent.
Does Either Sitting Have Any Other Advantages?
You should sit the test at the best time for you personally and when you will be best prepared.
For Australian students this is most likely to be the March Gamsat because it is preceded by the long summer holiday from December to the end of February which will give most people a reasonably long preparation period free from university course work.
For UK students the reverse is true and the September Gamsat will be preceded by the summer holidays.
Can you sit the Gamsat twice in one year?
Yes if you want you can sit the test both in March and again in September. In fact many students do this to try and improve their score at the second sitting since final offers of university places won't have been made by the time the September Gamsat comes around.
Whichever sitting you decide to take for the Gamsat the most important consideration of all is to make sure that the result from that sitting will be valid for the application cycle you are applying in.
For a detailed explanation of this and how long your results will be valid for both in Australia and the UK please see my other article How Long Are Gamsat Results Valid For?
For more help with GAMSAT check out Griffiths Gamsat Review Home Study System which takes you step by step through all three sections with advanced strategies for each.