I thought the song fit perfectly. It's straight up visceral metal, for one. And for two, it had some relevance regarding the COG army vs. the Locust and I'm willing to bet it has something to do with man creating the problem...hence the war pigs. Just a hunch.
Either way, I love me some metal and War Pigs is a classic and it fit the manly, "let's kick some ass" vibe that Gears conveys.