How to handle multiple years lost to test preparation on CV?
I agree with @socialsciencedoc's comment, although I think that what s/he describes is a more common phenomenon in the social sciences than in STEM fields.  In the former, it's rare to be a 20-something PhD, and in the latter it most certainly isn't: not getting your PhD in math until you're 30 makes you (slightly, and certainly inessentially) older than average.

Anyway, I am not really scrutinizing gaps in candidates CVs until after their PhD (at which point I am!).  Having a gap between undergrad and grad is still very common in STEM fields: e.g. many people do a couple years of high school teaching before the siren song of academia lures them back.  One of my current PhD students did some high school teaching (among other things...) and will be over 30 when he gets his PhD.  In his case, he acquired some skills in his "gap years" that are proving useful to him as a student and future mathematician.

Spending multiple years only studying for an exam is not something that most Western academics would be so familiar with, I'm afraid.  I'm not sure how to spin that positively.  So I would just not explain the gap at all.  Put your energy into what you can now control: getting a PhD in a timely manner (in math, four or five years is ideal; six is still totally fine; after that it begins to look like you lack sufficient locomotion) and, of course, proving some great theorems in your thesis work.  My eye would probably slide right over this gap in your CV...and I think that is exactly the way you want it.

Added: I missed this sentence reading your question the first time: " I do understand myself that my mathematical knowledge grew in that two years, still I can't find a single demonstrable item to support my case."  That's great; truly.  So your time was actually well spent, and I'm very glad to hear that.  You don't have to further justify that that time was well spent: it got you into the great university that you now attend.  So I think you have little to worry about, actually.