There are few universities that will accept your application for undergraduate medicine once you have already started another course. UTAS is the only one that I am aware of. Personally, once again this is only my opinion, I would base my decision of whether or not to take a GAP year and resit the UMAT on two things:
1. My ATAR, if my ATAR was good (i.e above median for those who get into monash, like 99.4 or more, or even down to 98.5 for a chance) then I would consider resitting the UMAT as improvement in the UMAT is likely to help me get the interview offer this time round. However if I did well in the UMAT, like 95 percentile or higher and my ATAR was moreso the reason that I did not get an offer - Then I probably would not resit the UMAT as the margin for improvement is low and I would not be able to improve my ATAR. So if your UMAT was your key weakness, and you personally feel that the next time round you can improve your UMAT score, then the GAP year would be a good idea.
2. This would for me be the clearest and most important factor - what my other plans for the GAP year are? Seriously, personally I would LOVE to have taken a GAP year and did some work, travel, anything, even a year of free time to myself to do absolute zilch, nada, nothing productive at all would have been awesome. Especially if you do end up getting into a course with 30-ish contact hours a week. So if you always had that travel plan to head off to Europe, parents to pay for it or a job offer to earn some money for it - whatever, maybe even a cadetship, anything at all, then go for the GAP year - they are never a waste of life, and seriously, they can be definitely worth graduating a year later.