Explosion can actually be a good move for Gigalith.
You might want it to set up Stealth Rock and then use Explosion. You might also want to use some support moves for your team.
Here is a good moveset that Gigalith can use:
In this case, you can use Sandstorm and Stealth Rock to set up. Earthquake is a good move, but does not get any STAB. Stone Edge, on the other hand, gets good power and STAB; but does not get good coverage and has somewhat low~decent accuracy. If you do not want to use Explosion because it KOs it and loses a sand support, Toxic and Iron Defense** are good substitutes.
Explosion is a good move for Gigalith, but it depends on the role you want. If you want to use it as a sand support (for your sandstorm team), go with one of the substitutes I mentioned for Explosion. If you want to use it as a Traplayer (a Pokemon that uses some arena traps and uses Explosion), go with Explosion.
I hope I helped!