I agree that there would be a fair mix of people who are aware, and people who are unaware to a degree. There's information mixing based on who your social circles are, and the VNPC population that the PC population rises out of can be pretty diverse and self-centered on their own struggle against the known.
I don't think hiding it would be unusual given how the gemmed are treated, but I think that it being hidden would make for a lot more fucked up reactions (having things kind of sprung on you like that can be weird, but settle into a nice internal conflict story) upon discovery. There's a lot of influences on how things go, and I consider the documentation to be the 'base position' for -most- starting PC's, but then with IC experiences shaping them from thereon out, with obvious filtered lenses applied as necessary to the character. I also think proud servants of the templarate would probably not hide it due to the company they keep (if it can be bent into an attempt to deceive in some fucked up way, the templarate/noble/upper crust crowd is not the people you want accusing you of such. Better to just take the treatment of gemmers than have it insinuated you're trying to fool them).
I don't think it would be odd to play someone from outside of allanak who is less aware of things, though I always find the ones who have no stigma attached to magick altogether to be a little jarring. Whether it be positive or negative, I think all cultures of Zalanthas have some idea of how they view magick, and as cabbage said, people with access to 'civilized, non-tribal' parts of the Known probably have at least heard of the gem, though it may be much different visually in their head. I do not, however, think that it should be instantly recognizeable to everyone all the time the further away from Allanak you get. I was involved in a tribal scene once where a gemmed was explaining their relationship to the gem with a small group of elves where they had their own magickers, and the idea of gemming them was a strange concept when the gem's purpose was described. Fun things.