Will preparing for the mathematics GRE help me towards my goal of doing research in applied mathematics? I did my graduation in electronics engineering from India, and an M.Sc. that concentrated on certain "sub-specializations" (communication, DSP and control engineering) from a UK university.

I was reading for a Ph.D. in the areas of machine learning and computer vision (also at a UK university), however due to a number of factors I was unable to complete it. These factors include health problems (addictions, hormonal imbalances, depression, etc.) and trying to work in a topic that was too mathematical for my background, in addition to having too much leeway (and consequently not enough support) in my research from my supervisors. (Evidently my supervisors thought me as sufficiently capable at the beginning of my research to let me do what I wanted, but eventually I proved them wrong! :P )

A few years have passed since I terminated my Ph.D. programme. I feel I have worked through most of my health problems now (not all the way there, but I feel better now than I remember being in a very long time). 



I did develop an appreciation for "pure" mathematics during my time doing Ph.D. research (the lack of sufficient knowledge and skill notwithstanding). Nowadays I spend some time studying undergraduate-level mathematics textbooks on my own. 

Basically I'm examining options that would prepare me to do research in mathematics applied to the area of machine learning, preferably in a professional or academic context (rather than just at a personal level). 

Studying on one's own is all very well, but I feel I need to impose more structure and have better focus. 

I recently found out about GRE subject tests. I was wondering if preparing for (and taking) the GRE mathematics test might be helpful? I'm looking at it from these angles: (a) having a study "syllabus" to help concentrate my effort, rather than jumping about subjects and topics haphazardly (b) getting an idea of where I stand compared to other students, and (c) strengthening my case in case there's a possibility of going back to university (at the postgraduate level, that is; while I might seriously consider doing an M.Sc. or M.Phil in applied mathematics in the US or the UK, I don't think it's practical to start afresh at the undergraduate level now.)

So, is preparing for the GRE mathematics test a good idea? Any other suggestions (alternatively, or alongside)? And - so as not to put the cart before the horse - how likely is it for someone from my background to be considered for admission to a postgraduate mathematics programme (at a decent university) in the first place?
The mathematics subject GRE covers roughly the first half of the content that one would learn in a good undergraduate math program.  Much of it (calculus, linear algebra, differential equations) is material that an engineering student would likely be familiar with already.

I think that the only good reason to study for the GRE is if you need to take the GRE, i.e. if you want to apply to a school that requires it.  In that case, it is probably best to spend a limited amount of time studying in the few months or weeks before you take it.  Otherwise, I think your time would be better spent on more advanced topics.