They are literally saying that some readers (those who are good at math) will already know that "Burma is roughly four and a half times the size of the capital of the UK" from what was written earlier ("and is 7,054 km²"). This isn't a serious statement about whether the readers are good at math or not, it's just a fun way of spicing up the sentence. This "as (the ___ among) you" structure is a way for the writer to interact with the readers a little. For example, "As you may know, I'm starting work next week." In this example sentence, "as you may know" is used to grab the listener's attention a little, or to add a little flair to the sentence. "mathematically-gifted" simply means "skilled when it comes to doing or understanding math" Hope this helps~

Hi. I myself is a second language English speaker but I will try to explain it as clear as I can. The word gifted in this case means talented or skilful at something. So the sentence is basically saying that if you are very good at mathematics, then you would have already known that Naypyidaw is four and a half times bigger than London as you have the ability to make such calculations from the information given. Hope this clears up any confusion.