According to the ESRB, this game contains: Violence
Parents shouldn't be too concerned. There is no blood or gore of any kind, although enemy units (mostly spaceships and tanks) do explode in a rather impressive display of colors and smoke. There is no sexual content and no questionable language whatsoever. It may be too difficult for your younger ones, but it won't corrupt their morals.
Gamers in general can expect a lot of straightforward shoot-em-up action and a nifty-looking Transformer wannabe. If all you want is some fast, no-nonsense action, then Gun Metal basically delivers. However, be warned that there's not much to the game (no hidden goodies that I'm aware of, no multiplayer, no extra modes) and that the second half gets frustratingly hard.
Transformer fans will want to check out the game just to play with the at-will transforming action, if nothing else. It's not an authentic Autobot or Decepticon, but unless Hasbro decides to bring the franchise into the next generation, the Havoc is as close as it gets until Robotech: Battlecry is released.
Hearing Impaired gamers will have no problems. All voice in the game is accompanied by text, and there's very little dialogue overall. Smooth sailing.