Except it's even more illogical than a hypersonic plane. "Whee I can have an element that can create entire gravitational fields. Oh yeah it's also abundant enough for us to be able to use it to plant people to the ground throughout a massive space station and is also totally logical in every way."
"No xkill you can't have a hypersonic plane when you've already attained Mach 4.7 in a plane forget you."
"Yes of course it'd be more dense because I should totally talk about atmospheric pressure when I have an element that can generate gravity."
Also, if you think about it, considering your Gravitanium is found underground, just what would happen if it was, say found in some place with incredibly high levels of pressure. Like, oh I dunno, below the crust. Or are you just going to say "oh well, uh, gravitanium can't be found below the crust because forget you"