Wow, I might actually play that if I ever bother to teambuild
I don't think I'll use Gigalith over Golem anyway. Golem's SR would lose some power, but Golem provides a lot more utility in STAB Earthquake, priority, and a Volt Switch absorber. Getting a bit more residual damage might not be important (unless there are calcs to prove it), and Golem will be much more useful midgame in comparison to Gigalith. Furthermore, I'm pretty certain that Golem would be able to fair better against several other stealth rockers. For example, it outspeeds and 2hkoes Gigalith, and earthquake + sucker punch would allow it to fair much better against Golurk than Gigalith would.
Rampardos, on the other hand, is a different story. I'm sure it's Attack stat is so much higher that the greater residual damage would actually matter. In addition, it has access to Mold Breaker, which means that in addition to setting up rocks, it would also be able to defeat other SR leads with EQ and Mold Breaker breaking through Sturdy. I'm not sure if unboosted EQ can OHKO Gigalith, but I'm pretty sure that slapping a Ground Gem on Rampardos would. Rampardos also would have greater utility later in the game because it can still threaten with Head Smash.
I also see Golurk becoming even more common because its SR is just as strong and it beats Gigalith and Rampardos fairly easily, in addition to all the positive traits it currently has