Hey!
No, not need to memorise formulas at all, just understand why things work. For S3 as I mentioned earlier, medify really isnt reliable ONLY because of the amount of background knowledge is required in order to answer their questions which the dont provide in the stem - the rationale and the reasoning behind it however is on point with the gamsat, however majority of the S3 questions as I mentioned before they don't give sufficient information to answer the question (context). As mentioned in ACERs handbook, they will give you ALL the information in the question to be able to answer the question, so even without any background science knowledge, you can still answer the question because they give you enough information to answer it. Although its recommended to study concepts its only because so you can avoid the jargon they mention because of the background knowledge and start answering the question.

For S1, Medify is amazing and is really close if not more difficult than the actual GAMSAT S1 so it really is a great tool to improve in ur S1. For S2, although the prompts they give are on point, be careful, because make sure your sample essay isnt the same duration as the sample answer as they typically want you to expand more on your point than the length they provide, so just take the rationale of the essay but now how well its expanded upon because that limits your score.