Trick Room teams would be great down here, probably about as anti-metagame as it gets honestly, and Duosion is definitely one of the most powerful abusers of it. He doesn't have the same staying power as his older brother, but he's one of the slowest and strongest Psychic types down here, and can at least put a hole in most of the tier.
Gig's best use is on a Trick Room team, where his virtually nonexistent speed and huge attack allow him to really shine. EdgeQuake can be dangerous behind base 135 Atk, especially if it's hitting you twice before you even have a chance to kill it. I also think Gigalith is much better than Rampardos in TR; no, he doesn't have as much Atk initially, but his great bulk and access to Curse allow him to raise all of his Atk, Def, and Spe all in one turn of TR (you'd only get three turns of attacking if you did this, but it's still something to consider).
I think if you're using him outside of TR you're better of with Regirock or Golem though; Gigalith just doesn't have anything to make him better than either of those 'mons otherwise.