No, it's a terrible move, and a really terrible move on Garchomp. First off, Chomp has Earthquake, which, when combined with STAB, hits 150 power (equal to unSTABbed Giga Impact) and Outrage, which, with STAB, hits 180 power (greater than unSTABbed Giga Impact). Continuing, Giga Impact is a terrible move in competitive that's only worth using if you're trolling, and even then you'll definitely find a better option for trolling out there (such as specschamp, who's classier). The reason? With Giga Impact, you give the opponent a free turn. A free turn for them to bring in something that threatens you and you can't ohko, like Balloon Double Dancer Terakion. And then watch it set up to +2. And then +2/+2 as you're forced to switch.

Get it?