But it isn't a myth. It's stated pretty matter-of-factly that the earth goddesses gave birth to the world, some gods built the world up from that primordial stage, and others established the nations of the world. No matter what, I don't think human belief would have been enough to retroactively create the planet.
Gil is apparently a facet of the Counter Force, so who knows if it would even do anything against him. Even if it did, I'd think the concept of ripping apart the world would sort of overpower it. If super powerful aliens can make the world their bitch, it seems likely a concept specifically meant to rip it apart would do the same.
Okay, I'm really trying to make this distinction here. The RM is holding a world up. The world inside is still a world, made of the same stuff as the outside (unless it is something freaky like Nrvnqsr's chaos). Attacking the world within the RM is not the same as attacking the RM itself. Crumbling the earth, sea, and sky of the world within is different than simply destroying the barrier to make it fizzle out. All of that is still going on inside the barrier, and if Rider was immortal and had infinite energy, it likely would have still held up even after Enuma Elish was done remaking it. Gil in CCC sticks it in the ground, and that seems to smash apart the area, so he probably could have done that to IH to simply end it if he had wished.