Gavrillium melts quickly, which is fine for some uses, but it probably won't work well in gunpowder---you need RAPID expansion of gas in gunpowder, and melting metal just won't cut it.
Also, a 200mm artillery gun is a little bit on the "Extremely oh-my-goodness huge" side of artillery pieces. Typical heavy artillery ranges from 120-150mm, light going from 75mm-120mm. No, there is no medium artillery, and yes everything gets smaller if you want a field gun instead of a howitzer.
Uh, Taricus? I was arguing against your statement about ballistic fiber-based armor being the only reasonable modern armor, not providing alternatives. You're also extremely, incredibly wrong about the ceramics despite the fact that I wasn't actually suggesting them as an alternative in any way---they're MEANT to shatter on impact. The shattering is what takes up the energy of the impact, preventing the energy from being transferred straight to YOU. Plate-carrier rigs with those plates are capable of turning a hit from a .50 cal sniper rifle (bullets capable of piercing ~1in of high-armor-grade steel) from "dead and your insides are now entirely on the outside" to "covered in a huge chest-spanning bruise, but not actually dead or even necessarily out of the fight".
I'm not sure we want to attempt something so insane with Gavrillium as you're suggesting, NUKE. Somehow extracting energy from the system like that is not only counterintuitive, but also makes it hard to imagine how ELSE we can use that particular ability of Gavrillium's. Also, if we stick to a "theme" for Gavrillium, i.e., it produces thermal energy and/or explodes, it might be easier to get more special functions out of it, since they're all obviously related and such. Oh, and it'll make it easier for us to consider ways to use the stuff.