re: In laymen's terms what is a GPa?
The "pascal" is the unit of pressure in the metric system. Since pressure is force divided by area, 1 pascal = 1 Newton/meter^2 , using the standard metric units.
It turns out though the Pascal is a rather small unit of pressure. But by a convenient coincidence it turns out that 1 atm is quite close to a power of 10 of pascals: 1 atm = (about) 10^5 Pa. A gigapascal (GPa) of course means 1 billion pascals, or about 10,000 atm, and 100 GPa is about 1 million atmospheres.
Robert Clark